Define big. Define small. It's all relative. Everyone has a channel which is smaller than them unless they have 0 subs.
So if a channel has 150 subs, why don't they collaborate with a channel with 1 sub? (They're small compared to a channel with 150) Probably for the exact same reason a channel with 150,000 subs doesn't want to collaborate with a channel with 1,000 subs.
Have you made any videos yet OP?