Size is not the issue, to me. Unlike many other things the Celtics do exceptionally well, their draft record has been average.
It's been a long time since The Cs have drafted an upside player, and succeeded. They've mostly been effective on the safe bet picks (i.e. 'hard nosed kid'), but they rarely takes chances on the upside of developmental players -- and when they do, the results have been weak.
Watching guys like Gobert, Giannis, DeAndre, Dray Green, Middleton, Jimmy Butler and a host of others slip by adds up -- especially considering the ones taken in their place, in many cases, left the league very quickly. Even something as simple as taking Hood over Young just didn't seem that tough to me, though I'm still hopeful Young could become an effective chucker in the league.
At some point, the excuses get boring, and there's enough of a track record that something's missing. The Rondo buy-and-pick was low risk, but truly genius. That was 10 years ago this summer. Besides that, you have to go back to Al Jefferson to find a guy who panned out to be an true All-Star -- that was 12 years ago this summer.
Yes, it's nice to have effective role guys through the draft, but there's a time and place for those picks (meaning, not every year) -- a squad of purely role guys won't get you far. In the modern NBA, trading for or signing premiere players has become more difficult, so you need to find stars through the draft.
They'll likely have a bunch more cracks at it again this summer....