I do have to say that I think you're making the same mistake with this sample that you always make when talking about small backs. You point out that a only a very small percentage of the players who get 1,000 yards after age 30 are "small backs", but you don't provide any context for that. What's the percentage of backs who enter the league who are small backs?
I looked into this a little more and some of the conclusions seem to support what I've been saying.
I went back and looked at all of the RBs picked in the first round between 1995-2004. I chose that range because it's fairly recent and most of the players in the sample are retired or irrelevant at this point. Only Steven Jackson and Willis McGahee have any real chance of additional production.
I used height/weight info to calculate the BMI for each player. I used precise measurements from NFL Draft Scout for the 1999-2005 numbers. I had to use less reliable measurements from football-reference for the other years because Draft Scout numbers aren't available for pre-1999 classes.
There were 30 backs picked in the first round between 1995-2004. The average BMI was 30.45. Five of the backs (16.7% of the overall population) had a BMI below 29:
Lawrence Phillips - 6' 212 (28.7)
John Avery - 5'9" 188 (27.8)
Napoleon Kaufman - 5'9" 185 (27.3)
Trung Canidate 5'10.5" 193 (27.3)
Warrick Dunn - 5'9" 180 (26.6)
I looked at all 30 backs in the overall sample and ranked them based on three main gauges of success: the # of 1,000+ yard seasons, the number of career touches (carries + receptions), and the peak volume season (the season in which the player had the most touches of his career). Here are the top 10 lists for each category:
MOST 1000+ YARD SEASONS
Steven Jackson (30.1) / 8
LaDainian Tomlinson (31.5) / 8
Jamal Lewis (32.9) / 7
Fred Taylor (30.9) / 7
Eddie George (29.4) / 7
Edgerrin James (29.3) / 7
Shaun Alexander (29.8) / 5
Thomas Jones (31.0) / 5
Ricky Williams (34.4) / 5
Warrick Dunn (26.6) / 5
Deuce McAllister (29.2) / 4
Willis McGahee (30.0) / 4
The average BMI for these players was 30.43. Basically identical to the average BMI of the entire group of backs. Warrick Dunn is the only small back to make the list.
MOST CAREER TOUCHES
LaDainian Tomlinson (31.5) 3798
Edgerrin James (29.3) 3461
Warrick Dunn (26.6) 3179
Thomas Jones (31.0) 2986
Eddie George (29.4) 2865
Fred Taylor (30.9) 2824
Steven Jackson (30.1) 2802
Ricky Williams (34.4) 2773
Jamal Lewis (32.9) 2763
Shaun Alexander (29.8) 2402
The average BMI for this group was 30.59. Higher than the overall average of 30.45 by a very slim margin. Once again, Dunn is the only small back to make the list.
TOP 10 VOLUME SEASONS
Larry Johnson (30.1) 457
Eddie George (29.4) 453
LaDainian Tomlinson (31.5) 451
Edgerrin James (29.3) 450
Ricky Williams (34.4) 442
Steven Jackson (30.1) 436
Deuce McAllister (29.2) 420
Jamal Lewis (32.9) 413
Fred Taylor (30.9) 393
Shaun Alexander (29.8) 385
The average BMI for this group was 30.76. Slightly higher than the overall population average of 30.45.
The high volume list shows a clear lack of small backs. You have to go down all the way to Napoleon Kaufman at #15 overall with 312 touches and Warrick Dunn at 16th overall with 309 touches to find an undersized RB. Their career bests are well below the career bests for the bigger backs. Kaufman only had two seasons of 200+ touches. Dunn had a whopping 9 seasons of 250+ touches, but never approached the staggering numbers of the best conventional backs. Tomlinson had 7 seasons of 350+ touches. Eddie George had 6. James had 6 and another with 348. Alexander had 5. Ricky Williams had 3. Steven Jackson has 3. Jamal Lewis had 2. Deuce had 2. Larry Johnson had 2.
So while BMI doesn't seem to have any significant effect on the expectation for a prospect's career 1000+ yard seasons or career workload, the numbers from this sample suggest that small backs are far less likely to have high volume seasons. Backs below the 29 BMI threshold accounted for a very small percentage of the high volume seasons yielded by this group of 30 players even though they represented almost 17% of the total population. If you set the cutoff at 350 touches, small backs account for 0% of the seasons. If you set the cutoff at 300 touches, small backs account for 4.5% (3 out of 67). You have to drop the cutoff down into Dunn's wheelhouse of 250-290 touches before small backs make any kind of a dent. Even then, they lag well behind the bigger backs.
So with very few exceptions,
small backs don't have high volume seasons. That's a problem in FF because those high volume seasons are extremely valuable.
One way to look at the numbers is to balk at the sample size and say small backs are underrepresented because there simply haven't been enough of them to get the right results. There's probably some truth in that. There have been some small backs entering the league in the years since the 2004 cutoff and some of them have had success. Chris Johnson has been a groundbreaking player. He has two 350+ touch seasons despite a rail thin frame. Jahvid Best, Reggie Bush, CJ Spiller, and Darren McFadden haven't fared quite so well.
The biggest elephant in the room is Adrian Peterson, who's a small back if you go by the BMI numbers alone. Subjectively, I don't think anyone has ever cited a lack of size or power in his game. I also think there are some limitations to BMI as a gauge of size. I think overall height and weight play a role. For example, you can't convince me that Dion Lewis at 5'6.5" 193 (30.7 BMI) runs as hard as Steven Jackson at 6'1.4" 231 (30.1 BMI). My hunch is that a slightly thinner, taller back will run with equivalent power to a shorter back with a slightly higher BMI. I think that partially explains why taller and slightly thinner backs like Jackson, Peterson, and Forte are able to run with good power and handle a high volume of carries despite not scoring that high in the BMI scale.
Even if you allow Peterson in the sample, the absolute best of the objectively small backs (Peterson, Chris Johnson, Dunn) have not come close to the career bests of guys like Ricky, Edge, and Tomlinson in terms of maximum volume in a single season. I think it all points towards the conclusion that small backs can have good or even great careers, but if you're looking for someone who has 350-450 touch per season upside you're probably better off focusing on more conventional backs. Even good players like Jamaal Charles, CJ Spiller, and Reggie Bush lag behind equivalent big backs like Doug Martin, Ray Rice, and Arian Foster in terms of volume. The only way they can match their production is through a higher YPC.
If you went into this post thinking size doesn't matter at all then you probably aren't convinced otherwise, but I think there's some decent support for the idea that thinner backs (especially short and thin backs) are not as likely to yield the 350-450 touch seasons that are so crucial to getting difference-maker production at RB.