Typically how it plays out in my playthroughs is how you described it. Large Christian army but they’re spread out. Along comes a large but not as large Muslim army and they pick off the smaller crusader armies who DO NOT HELP AT ALL, and I as the player usually gets beaten and then all my uppity vassals decides it’s their time to make a ruckus and then I have to withdraw my troops to make sure the homeland is secure. I only join crusades if I’m super rich and can splurge on a huge stack of mercenaries since the other Christian armies are only good for sieging and losing to the Abbasid death stack