What really brings points is losing men. Engaging a big battle and losing is granted to boost your contribution. Don't fuck up the crusade, though, it better be worth it.
Played as Bouillon, had my 6k stack with mercs already in the adria when the crusade triggered. Started with comfortable 70% contribution when others had came in. England engaged the Fatimid stack and lost the battle, overtook me in contribution, so I took my attached counts, engaged too, lost also, but managed to push my contribution back up. Then right away re-engaged with the english together and defeated them.
On a sidenote, as it wasn't mentioned, it is not the sieges themselves giving contribution points to the crusade (they give warscore).
What "contributes" is your armies presence in the destinated crusade target- something which lasting sieges typically provide. If you observe the ticket on the contribution score on the crusade flag, you'll notice that the contribution level of all leaders who have soldiers in the crusade target area raises daily- so yes, having your army in spot to land when the crusade triggers is a valid tactic to be biggest contributer as small fish.
Large battles bring more, however- if an enemy stack you can engage passes by closely, don't hesitate, as long as you are fighting crusade target or defenders in battle, you also get contribution daily, regardless of the location. And if you contribute a serious army, you get much more contribution than sieging can give this way.