Why is my transaction so big and thus so expensive?
I think you answered your own question.  Yes, the transaction is so expensive because you are combining multiple inputs.  This makes the transaction high in terms of bytes, but low in terms of value.  As such, the transaction fee (which is based on bytes, not value) may not be worth the transfer you are attempting.

However, since it sounds like what you are actually trying to do is access the BCH (Bitcoin Cash) that you also own due to the fork, you can instead leave your bitcoin where it is on the main Bitcoin blockchain, and instead perform your transfer on the Bitcoin Cash blockchain where the fee will likely be cheaper.  This is, of course, assuming you need to move your bch at all.  Unless you need to spend it, you can just leave your bch where it is as well.