Bitcoin Cash Clarification and Bitcoin SV

Fairly new to Bitcoin Cash and I am getting sold on it. I'm not sure why things went the way they did through the hard fork. Now from what I hear, the downside increasing block sizes comes with a drawback in security. How serious is this drawback and has there ever been any issues?

Now learning about Bitcoin SV.. Further expanding the block limit and perhaps rendering the chain unsustainable by decreasing the incentive to sell hash power to the network. Plus the multiple 51% attacks the network encountered. Did BCH find the optimal blocksize for current times?