[[[ I'm a tall woman, but I still want to carry around a 15 pound instrument. ]]]
Do you mean, you are tall but you still do NOT want to carry around a 15 pound instrument?
12X4 may be heavy for you because 48 bass accordions in the 12X4 configuration are usually the inch or cm dimensions of a 34-key 72-bass instrument.
Don't know about 32-bass, but plenty of two-voice MM 26 treble/48 bass in 8X6 weigh around 11.5 pounds. You can make a lot of music with a 26/48. If you want 12 bass tones, a 26/60 [12X5, no diminished row] would give you all 12 bass notes, and a two-voice MM in that config would also weigh 11-12 pounds. Hohner until recently made a Hohner Bravo II 60-bass MM weighing 11-12 pounds. I think they aren't putting out the 60b anymore, but there are still a few purchasable online.
I endorse Weltmeister in this weight/size range. The Welt Perle 26/68 MM is around 11.5 pounds, and in the exact same dimensions and weight, they give you a 30/60 MM, the Rubin, with slimmer treble keys to get 30 in there. These models are so light partly because Welt gives them a 3-voice bass, not 4. Personally, I don't need 4 bass voices, and think this is a great idea.
Weltmeister also does an unheard-of LMM 5-switch at thirteen pounds, same dimensions as the Perle and the Rubin, but a 30/72. How do they do it? The bass is only 3 voices, not 4.
If you want Italian and pricier, some of the Italian makers have woken up to the fact that there is a market out there for pro-quality craftsmanship, reeds, and components in small models, and are putting out some very nice 26/48s. SEE the Serenellini 262.