Difficulty strumming concert scale uke

I've got two ukes I'm starting to play for the first time in years. One is my grandpa's vintage Martin soprano. The other is a Triumph (Northern clone) that I'm pretty sure is concert scale. The Martin being my grandad's (and a Martin) I decided to get the Triumph to have something to learn on. The trouble is, compared to the soprano, the Triumph is like strumming a cheese slicer. If I use a pick or my nails, it sounds great. If I try and play with my finger like I do on the soprano, I wind up with a sore finger and poor sound. Left hand work is just fine.

I've got 5 year old Aquilas on the Triumph concert and 10 year old Martin M600s on the Martin. Is it the type of strings, the scale, the age of the strings, or me being a giant wuss that makes the larger uke more difficult/painful to play?

I've searched and searched for this, so if I missed something someplace obvious please point the way.

Not sure about the wuss part but i'm pretty sure that there is more tension on the strings on the concert scale that makes quite a difference in the strumming. I find Martin strings very comfortable to play so maybe you can try switching to a set of M600 on the concert and see if it helps. Another option may be to take a look at your technique and try to back off a little in the attack and how hard you strum, a softer strum may be less painful in the longer term.

first off, both need new strings!

I do find Aquilas a little harder to play (too stiff and grippy) and much prefer the feel of fluorocarbon strings. Worth CLs are very light soft strings that would feel better. On a nice uke they'll sound better too. Not sure how they will sound on the Triumph.

It sounds to me like the strings and the action may be causing issues.
Is the string height at the 12 fret different on the ukes?
If so that may be the root cause of the feel.

:agree: New strings and check action

Edited to say Thanks!
The action is a mile high since this thing is a 30 dollar student model made to have the stuffing knocked out of it. I'll try and give it a proper setup someday, but aside from feeling a little rough, tone has always been fine, if a bit quiet. I don't know why these don't get more love as an entry level uke.
I put some Martins on it, since it's all they had at the store closest to me. It's so much more comfy to play now. We'll see how the tone is once these get settled in, but they feel much better, if a bit quieter.

I have an old Triumph, and I've found it to be a very quiet instrument. Nice for practicing at low volumes, but it definitely doesn't "sing" like most of my other ukes. So maybe you're unconsciously strumming too hard, to compensate for its low volume. Just a thought.