Mulder is 6'8" & 225lb per Ft Wayne's website. So yes he is on the smaller end of post players. But in the highlight video I watched he looks to have a great motor with decent quickness & explosion for a post player.
But what is a Big 12 post player? In watching the better Big 12 teams this year I feel coaches like a 2 headed monster. An uber athlete shot blocker and bruising rebounder. Here are the top Big12 teams post players:
Houston: Francis 6'8" 235lbs / Tugler 6'8" 230lbs / Roberts 6'8" 235lbs
TT: Toppen 6''9" 225lbs / Federiko 6'11" 215lbs
Arizona: Veesaar 7'0" 235lbs / Awaka 6'8" 250lbs
BYU: Keita 6'8" / Traore 6'8"
KU: Dickinson 7'2" / Bidunga 6'9" 220lbs
So it doesn't seem like Mulder's size would be a liability. But it probably makes sense that our other post be taller and more of a shot blocker type like Ward & Osunniyi.
Now list verticals of all those guys