I'm curious what gives you this impression. I haven't watched them a lot, but as far as I know they usually have either Hollingshed or Miller on the floor (maybe both together a little bit) surrounded by 4 guards/wings. The Buffs aren't one of the taller teams in the Pac 12.
I have only seen them once and not the whole game, so I can't tell you about the rotation and the minutes but it made look at their roster and they have a lot of tall players and upperclassmen....I figured that they are not there to sit but are part of the rotation over a season. According to their roster:
Upperclass (Junior to 5th Year): six 6'0" or taller, 2 x 6'1" and 3x 6'3";
Underclass (F, S): four 6'0" or taller, 1x 6'0", 2x 6'3" and 1x 6'4"
That's 10 of the 15 on the roster and one of the guards is 5'11", a sophomore.
The rest? 1x 5'7" (Jr), 1x 5'8" (Soph) and 2x 5'9" (Sr, Fr)
They have four guards 6'0" (2), 6'1" and 6'3" to go along with the above.
All those designated as forwards are 6'1" or 6'3"
The centers are 6'3" and 6'4"
Both Hollingshed (forward) and Miller (center) are listed as 6'3"
Anyway you cut it, that's a lot of height wasted (including lots of upperclass players), if all the under 6'0" guards are used, including freshmen and sophomores when two of the over 6'0" guards are seniors and the other a soph. Unless, of course, the teams best guards and wings (would have to be guards playing wing and PF, since you said the 5th is either MIller or Hollinshed, not both) are all under 6'0".
There's another possibility: the posted heights are not correct?