Here is a whole topic about the assumption you are making.
FJ is making a different argument than you are but it essentially boils down to the same thing, people do not think smaller RB are durable enough to handle massive workload.
The whole size or weight related ideas are difficult to pin down because there are so many different variables that go into a RB opportunity to be a "featured" RB and people have different definitions for this as well clouding the issue.
If a player does not touch the ball much, but stays healthy it isn't really the same thing as a player getting a lot of touches and staying healthy. So I think a number of rushing attempts should be one data point to consider when looking at how players have actually performed in the NFL, which can then be cross referenced with their weight.
Research sample For single seasons, from 2000 to 2017, played RB, requiring Rushing Att >= 200, sorted by descending Weight
The reason I use 2000-2017 is because PFR has combine data from 2000 but not before that. 200 rushing attempts in a season isn't a very high threshold, but it does a good job of showing a player who did get the ball a lot and was able to sustain that over enough games to at least amass 200 rushing attempts.
200 rushing attempts is not always a good season for a RB. Mileage varies, but at least it is a lot of carries. Enough carries that the guy might get hurt due to attrition of multiple impacts that a player with fewer touches would have less risk of.
This sample is incomplete, as many of the players on the list were drafted before 2000 and PFR does not have accurate combine data of those players from before 2000. So players such as Emmitt Smith who was 210 lbs at the combine is being listed as a higher weight than this, because Smith added weight after he entered the NFL. Also the older players may have had more 200 rushing attempt seasons prior to 2000 that are not accounted for here.
Although 215 lbs is arbitrary, we have to choose some cut off point to differentiate heavier from lighter RB.
Using 215lbs 269 out of 396 players in the sample were 215 lbs or higher. So about 2 out of 3 RB weighed 215 or more lbs.
This by itself shows that it is more likely for a heavier RB to get 200 rushing attempts than a player who is lighter than that. That is a fact of history. That doesn't mean that a lighter RB is more likely to be injured though, it just means the majority of the players who have had a 200 rushing attempt season are heavier than 215
One way to look at if the player had durability is to look at if they had more than one season with 200 rushing attempts. There are a lot of one year wonders in this sample. While this ay be due to benching and a lot of other reasons besides being injured (its not Chester Taylors fault the Vikings drafted Adrian Peterson after only giving him one year to be their feature RB for example) a player at bare minimum needs to be healthy for 10 or more games to get 200 rushing attempts.
The number of players over 215 lbs who only had one 200 rushing attempt season was 35. 13% of them. For the RB lighter than 215 there were 12. 9% of them. So it is slightly more likely for a heavier RB to only have one 200 rushing attempt season than a smaller one will.
If anyone wants to look at how often each of these players were injured, I will list the players who had two or more 200 rushing attempt seasons.
RB 215 or more
Brandon Jacobs 3 seasons
Jerome Bettis* 4 seasons
Eddie Lacy 2 seasons
LeGarrette Blount 2 season
Jamal Lewis 8 seasons
Jonathan Stewart 3 seasons
LenDale White 2 seasons
Kevan Barlow 2 seasons
Michael Turner 3 seasons
Eddie George 4 seasons
Shonn Greene 2 seasons
Fred Taylor 6 seasons
Ronnie Brown 4 seasons
Deuce McAllister 4 seasons
Mike Anderson 2 seasons
Stephen Davis 4 seasons
Jeremy Hill 3 seasons
Latavius Murray 2 season
Carlos Hyde 2 seasons
Ezekial Elliot 2 seasons
Steven Jackson 8 seasons
Larry Johnson 2 seasons
Willis McGahee 5 seasons
Anthony Thomas 3 seasons
Arian Foster 4 seasons
Todd Gurley 3 season
Ricky Williams 5 seasons
Shaun Alexander 7 seasons
Le'Veon Bell 4 seasons
Corey Dillon 5 seasons
Jordan Howard 2 seasons
Alfred Morris 4 seasons
Doug Martin 2 seasons
Cedric Benson 4 seasons
DeShaun Foster 2 seasons
Marion Barber 3 seasons
Jay Ajayi 2 seasons
Matt Forte 9 seasons
Emmitt Smith* 4 seasons
LaDainian Tomlinson* 10 seasons
Justin Fargas 2 seasons
Rudi Johnson 4 seasons
Thomas Jones 7 seasons
Ryan Mathews 2 seasons
Lamar Miller 3 seasons
DeMarco Murray 3 seasons
Adrian Peterson 8 seasons
Chris Brown 2 seasons
Ryan Grant 2 seasons
Michael Pittman 3 seasons
Ahman Green 6 seasons
Cadillac Williams 3 seasons
Domanick Williams 3 seasons
Melvin Gordon 2 seasons
BenJarvus Green-Ellis 3 seasons
Garrison Hearst 2 seasons
Travis Henry 4 seasons
Mark Ingram 3 seasons
Fred Jackson 3 seasons
Marshawn Lynch 8 seasons
Knowshon Moreno 2 seasons
RB 214 lbs or less
Edgerrin James 7 seasons
Priest Holmes 3 seasons
Frank Gore 12 seasons
Marshall Faulk* 4 seasons
Joseph Addai 3 seasons
Julius Jones 2 seasons
Curtis Martin* 6 seasons
LeSean McCoy 8 seasons
Darren McFadden 3 seasons
Rashard Mendenhall 4 seasons
Kevin Smith 2 seasons
DeAngelo Williams 4 seasons
Willie Parker 4 seasons
Reuben Droughns 3 seasons
Devonta Freeman 2 seasons
Maurice Jones-Drew 4 seasons
Clinton Portis 6 seasons
Tiki Barber 6 seasons
Reggie Bush 3 seasons
Brian Westbrook 3 seasons
Jamaal Charles 4 seasons
Ahmad Bradshaw 2 seasons
Chris Johnson 6 seasons
Ray Rice 5 seasons
C.J. Spiller 2 seasons
Charlie Garner 2 seasons
Warrick Dunn 6 seasons
While there are certainly some very good RB in the 215 lb or more group (there should be since it is 67.9% of the whole group) there are some epic RB careers from the lighter group. That is my view makes the assumption that smaller RB can't be feature RB dangerously myopic.
But to each their own.