Republicans can’t beat a Democratic Senator in a Trump +42 state while the last blue state Republican with a history of winning absolute landslides is trailing in a Clinton +3 state. I’d say any "asymmetric electoral polarization" in blue vs. red states more than negates the Democrats' "small state problem" in the Senate.
You don't think it's a determining factor that 2018 was a huge Democratic year and 2020 looks to be better? I'm biased because I live in the Obama+30 (or so, I'm not looking it up) state that voted for Scott Brown, but I bet that come 2022, if Biden wins, we're going to see more Scott Browns and Mark Kirks winning in states that easily went went to Biden.