for some reason they want to get away from 40 man raids... at least in PVE... could be due to frame rate issues, but look at alterac valley, went from 40 to 25 man. 25 man raids are basically 25 people at same difficulty as 10 man... just more people. world bosses are 5 man now (though you can make a 40 man raid for it if you want)
either the point is to cut down on so many people and spell effects in an area, or something to do with difficulty and possibly scaling/mechanics and damage and health numbers becoming so big fights become incomprehensible... ie. 500 dmg hit on a 10,000 health boss is easier to wrap your head around whats happening than a 50,000 dmg hit on a 1,000,000 health boss....just an idea
edit: oh, and I forgot....
if a world boss is designed for 5 players, you can get 5 or 10 extra and take him down, if its scaled and balanced for 25 players , say max level in epic gear, you'll need much more, say 30-40 players to take him down... logistics of finding 40 players to raid and coordinate successful with is very difficult and unlikely, as seen in vanilla. finding 5-10 players to raid with is much easier