One possible culprit is anything that monitors buffs and debuffs. This is something of a pain for healers.
When you have 10 people if one person applies a buff then this will be recorded 10 times. If 10 people bring a buff each thats 10 buffs being applied 10 times totalling 100 events.
If you have 25 people each bringing one buff thats 25 buffs applied 25 times. Thats 625 events.
There are 26 types of buff and debuff involved in raiding. All are brought by multiple specs. Chances are a 25 man will have them all, probably multiple versions of the same thing - ie horn of winter and battle shout.
This is on top of there being more players dishing out damage, receiving damage, being healed and healing people,
Then you have more than twice the number of characters and spell effects on screen.
Its very demanding on your system, especially during bloodlust when everything happens that bit faster and everyone is all big and shiny.