Does receiving multiple times to the same address reduce the tx size when sending money from that address?
I assume not, because transactions refer to other txes rather than addresses. But maybe there are still opportunities to optimize something away?
No, it does not. On a technical level, addresses do not exist. What exists are transaction outputs. Receiving 100 times with 100 transaction outputs associated with the same address is no different from receiving 100 times with 100 transaction outputs all associated with 100 different addresses. When you spend those 100 outputs, the fee will be the same regardless of which addresses you used.