In Wrath, 10 man, both normal and heroic had lower ilvl and gave less loot. 25 man, both normal and heroic had higher ilvl and more loot.
Cataclysm, Blizz made both the same ilvl, 10 man just gave less, but Blizz screwed up the tuning, and I don’t know if it happened on other servers, but on mine, a lot of 10 man guilds that thrived in Wrath slowly died out throughout Cataclysm.
Plus I remember a lot of players crying they had to run the same raid 4 times to maximize gearing. Most the 10 man guilds I knew never worried about that though so might have been a min/max player thing.