Uh, Dave … you don’t seem to be especially familiar with Ellis or Chavez. First of all, yes, both were failed shortstops — but IIRC, the last time Chavez played short in anything other than an emergency-sub situation was high school. Ellis came up through the Royals system (so it wasn’t the A’s who determined by his body type that he should play 2B) playing both SS and 2B about equally (so the Royals didn’t sort him to 2B), and when he was acquired by the A’s, they had Miguel Tejada playing SS (so the A’s only sorted him to 2B out of roster considerations). What’s more, Ellis’s throwing shoulder was destroyed by a collision with Bobby Crosby the year after he was acquired; he’s made a remarkable non-surgical comeback, but his shoulder is no longer capable of making the throws from SS or 3B. What’s more, yes, Chavez is taller — but he’s also put on a lot of weight since coming up to the majors–and as a third baseman; and I suspect that weight profile is similar across a lot of third basemen (of course, part of the sorting you hypothesize could be selecting for guys with frames that can support more body mass).

The overall pattern you identify may be true, but (a) it’s more complicated than you describe, and (b) Chavez and Ellis aren’t the best tandem to illustrate it.

I’m not sure Chavez couldn’t play 2B, nor that pre-injury Ellis couldn’t play 3B, but each would have a high bar to clear to be as good as they are at the current positions, as well as the other is at his current position.