You keep mentioning this- Charles doesn't have the "bulk of a conventional three down back". Hogwash and poppycock. Charles plays for the worst team in the league, his coaches have admitted that they had no idea how few carries they were giving him at times, and he still ranks 8th in the league in carries. He's on pace for 300. Trent "bulk of a conventional three down back" Richardson has a whopping seven carries more than Charles. You need to give up the ghost, here. Maybe in your grandma's NFL, guys who look like Charles don't get huge workloads, but this isn't Granny's NFL. Highly drafted, first team AP All Pros like Peterson, Chris Johnson, and yes, Jamaal Charles actually get stereotypical, prototypical three-down RB workloads now.
You better get on the Spiller train while you have the chance, too, because despite your antiquated biases, he's hopping on the 300 touch train starting next season, too. Choo choo!
Five years into his career Charles hasn't even come close to the 300 carry barrier. I would draft players like him and Spiller because they have dynamic big play skills, but not because I ever expect them to log insane volume. The fact that guys like Richardson and Martin are putting up touch numbers that would be career highs for guys like Charles and Spiller as rookies kind of illustrates why, all else being equal, I favor that kind of back. 9 of the top 10 NFL leaders in rushing attempts are right in that 215-230 pound sweet spot.

Charles gets a mulligan for year 1. He was drafted to sit behind Larry Johnson, and as a rookie, that's what he did. He also gets a mulligan for year 4, because he tore his ACL, and you have yet to demonstrate any link between size and torn ACLs, making it a non-predictive event (Jamaal Lewis, Terrell Davis, Rashard Mendenhall- way more ideal backs shred their ligaments than small backs). That leaves us with three seasons. Carry totals? 190, 230, 295 (pro-rated). Total touches? 230, 275, 332 (pro-rated). Why, what do we have here? It looks to me like we have a bona-fide workhorse back! He wasn't one from day 1, but neither were ideal backs like MJD, Rice, Priest Holmes, Stephen Jackson, Jonathan Stewart, Rashard Mendenhall, etc, etc, etc. Or, for that matter, neither were non-ideal backs like Chris Johnson, Brian Westbrook, Warrick Dunn, etc, etc, etc. Doesn't change the fact that he's been trending towards workhorse status for his entire career, and now he's finally reached it, and anyone with eyes could have seen it coming from miles away if they weren't so hung up on their own biases and preconceived notions about ideal body types for workhorse RBs. Face it, you were sleeping on Charles's ability to handle a heavy workload, just like you slept on Chris Johnson's, and just like you're sleeping on Spiller's.
Fun fact! Jamaal Charles has essentially as many touches per game over the last four years (17.22) as your pet favorite "ideally sized" mediocrity, Rashard Mendenhall (18.08). All this hue and cry you keep raising about how Mendenhall is a proven workhorse with multiple consecutive workhorse seasons and is an ideal size and that's the kind of guy you like to bet on, while Charles is this undersized, sub-ideal CoP back who will never get a real workload and anyone who thinks otherwise is deluding themselves... yeah, all that drama is over what essentially amounts to less than 14 touches per 16 games. If that's what "ideal size" is worth- 14 extra touches a year- then ideal size isn't worth wasting bandwidth debating over. Increasing yards per touch by just a single tenth will have more than double the impact of increasing total touches by just 14 a year.