Ah, but you have to remember that Blood and Crip gangs aren't one cohesive organization. It's more like a federation really, but theres alot've infighting. See, you can set up a gang on 13th avenue in LA or something (hypotheticly) and you'll have to choose which side to go into because of the gangs that surround you (maybe they're all crip affiliated, or sureno affiliated) and you'll just join that one. Not to mention Crip sets are notorious for in-fighting, it's rumoroued that crips are more responsible for other crips dieing than the bloods combined.

Then again the bloods sometimes fight each other, from what I've read though it's supposedly less than what the Crips pull off, maybe because of size, there aren't as many Blood sets as there are Crip sets.

Then again thats a characteristic of street gangs in general, infighting. Ever since Jeff Fort got canned Blackstone has kept going downhill.

So, numericlly they may have more members, but its also far far easier to join/leave a gang. There are no "books" to open, no Red poles who have a set quota of members they need, you have a ranking system but really theres no set limit on membership. Most of the time potential recruits are coerced into joining, that could probably explain it as well.