But wouldn't that increase the KB size (and fee) of a transaction if I send it all from the different subwallets? Like the additionals coin control fee.
No. Bitcoin does not actually work with addresses or balances. It works on transaction outputs. Every time you receive a payment, it creates a transaction output. When you want to spend you spend those transaction outputs. Receiving 100 payments to one address creates 100 outputs, likewise receiving 100 payments to 100 addresses is still 100 outputs.