Both the 13" and 15" are supremely portable. Pick up a laptop from 10 years ago and you'll realize just how portable these are. We didn't all have gym sessions back then for our laptops either, we just used them and carried them.
All the same, I prefer the 13". I'm getting older and considering the 15" for my next one but more likely I'll stick with 13". I'm out of the home about 5 night every two weeks and typically take my laptop with my in my backpack, and space is a bit of a premium. It's not a huge difference between the two though.
I have a second screen at home but rarely use it. The 13" is just fine for me.
In reality, both are super portable.