Yes but bigger blocks could mean that it's more expensive for miners to process the blocks.
Sure the extra validation/processing cost pales into insignificance compared to the cost of finding the POW?
Yes, validation/processing cost itself small compared with cost of finding the POW. But you need multiply it by number of all nodes in BTC network.
There are a lot of miners too, but cost of finding the POW is split between them. Each do only small part.
While validation/processing cost do not split -
each BTC node need to "pay" it.