It is pretty obvious that new dungeons pretty much equals smaller raids. In TBC there was 1 new dungeon, along with a 5 boss raid. WotLK launched with only 2 new bosses (Sarth and Malygos) along with scaled up old raid. Then we got the amazing raid that was Ulduar. After that we got 1 new dungeon that takes place in a single arena, along with a 5 boss raid that takes place in that exact same arena plus an extra room for the final boss. We then got 3 new dungeons and a large raid. But the only reason we were able to get those new dungeons was because they put very little resources into Naxx/ToC.
Now moving on to Cata. It launches with 3 raids with a total of 13 bosses. After that we get a rehashed ZA and a completely remade ZG in 4.1. In 4.2, we get a 7 boss raid. And then to finish it off, we get 3 new dungeons with an 8 boss raid.
Now lets look at MoP and WoD. No new dungeons throughout both expansions, but all raids had 12+ bosses.
TBC
Raid Bosses: 41
New Dungeon Bosses: 4
Total Bosses: 45
Wrath
Raid Bosses: 34 (excluding Naxx)
New Dungeon Bosses: 11
Total Bosses: 45
Cata
Raid Bosses: 28
New Dungeon Bosses: 17 (excluding ZA)
Total Bosses: 45
MoP
Raid Bosses: 43
Total Bosses: 43
WoD
Raid Bosses: 30
Total Bosses: 30