I was actually surprised in the Minnesota game (the highlights are in his church thread) he posted up on Shabazz Muhammed (he's a legit SF type-body and build like Simmons) and made an easy shot, he also posted up Bjelica a 6'10 guy and got all the way to the rim. These guys are not good defenders, but they are tall/strong guys. In no way midgets. I also remember him shooting hook shots against Jokic in that Denver game. He has such good ballhandling that he will get to his spots unless you get physical with him. If you get too physical he might draw fouls.. He's actually very skilled in the post.
I don' think they post him up enough bc he hasn't earned it? gotten over himself? a mix?. They also haven't needed him to. December of last season, his rookie year, more raw than he is now, undermanned in the bench, Pop asked him to post up. He scored 15 points in that game, mostly self generated shots and drew fouls even on a legit SF in Nic Batum. For sure he can post up and has game. Spurs just don't ask him to bc they have better options there. They day they need him to do that, they will and I think he will come through for us like he did in SL. If you think about it the kind of game they have him playing (spotting up for 3) is not the game they asked him to develop (what we saw in SL).
There will come a time when we will need him to play aggressive and make stuff happen. Maybe next season with different personnel. It's not right now, and maybe Pop even wants to get him in a defensive mindset right now and work on other holes in his game, like his 3 pt shot before giving him a better role.