That because two different teams worked on those two episodes. Dan Curry did Q-Who. ANd Dan's work is always overlite, and isn't as sharply focused. Never has been. Rob Legato was lead on Best of Both Worlds I & 2. Also didn't hurt that as time progressed both producers learned how to better managed the facilities they had, and replacing the 6footer allowed far more time to be fred up for other shots.
Also Paramount was willing to give them a bigger budget because BOBW was the first two parter on the show so far.
Didn't Q who mainly use the 2footer?