Here's a few that I remember:
1) An episode of Family Matters that in the first 2 or 3 minutes (if I remember correctly) has Laura walking down the stairs in an obviously padded bra. Steve Urkel then says something like "Laura, you've got bazooms!"
2) In an episode of Sister Sister, one of the twin females had on a padded bra. The male co-star said something like "Damn, have you been eating your vegetables?"
3) The Seinfeld episode that guest starred Terri Hatcher. The quote when asked if her breasts were real was "They're real and incredible"
4) In an episode of Living Single, Kim Fields' character got a breast reduction.
5) There was this one show that I think came on Fox in the early mid 90's about 3 plump women who were roommates/friends. One of the episodes guest starred Dolly Parton. Somebody said to one of the ladies, "What could you possibly have in common with Dolly Parton?" They all looked down at their breasts. It was hard to guess the bra size of the women due to their overally size, but they were at least a D/DD or so.
6) The Charmed BE episode (I know's it not a sitcom)
7) There was a series on the WB (I think) that had Kirk Cameron taking caring of his younger brothers/sister. He went shopping for his sisters first bra.

Punky Brewster. There was the episode where she said to Henry, "I'm getting boobs".
9) Night Court BE episode. There was a character on this episode that could do magic tricks, similar to Judge Harry. He said something to Dan Fielding, and Dan then starting growing boobs (balloons under the shirt). He said "I've been boobie trapped!"
10) Hype. During this sketch comedy type show on the WB, they had something that made womens breasts grow/implants, or something like that. I only saw it once and didn't catch all of it.