I'd consider the 2019 15" to be quite portable already. It weighs next to nothing and is think with the only really "big" part of it being its footprint. Even that's not that large compared to-say-a 13" compact laptop from 10 years ago.
In any case, if the 15" wasn't portable enough for me, I'd personally seriously evaluate whether or not it's the right system to have. To me, the appeal of a 15" MBP has always been that it's a lot of power and always a good screen(although the definition of "good" has changed over the last several years) in a package that's still portable. The 13" MBP models, along with the MB and the MBA, sacrifice performance in the name of portability, although for many people these models are more than enough for their main/primary computer.
If I already had a 15" and wanted something smaller for travel, I'd seriously consider returning it and replacing it with a pair of computers-an iMac and an MBA. The iMac gives you a lot more bang for the buck in terms of performance than any portable model does, while the MBA gives you portability when you need it.
I have to admit always hold a bit of prejudice against the MacBook(in its current incarnation), and the current MBA makes me dislike it even more. The MacBook basically is designed for portability above anything else(but aside from thickness, I still think the long gone 11" Air was better in that regard), but is still $100 more than the not-much-larger 13" rMBA, while the latter is a much, much better specced computer even in its base model form. For a while before the introduction of the rMBA, the MB filled a hole for an ultra-portable with a super high res display(at a cost) but in my mind the revised MBA is pretty much in every way better aside from being ~12 oz. heavier and .16" thicker.
