The only time I have ever had a GFX in my shop was when I was working on one for someone. I use the Battery Wizard, which I designed, build, and sell. And I have found that higher Ah packs have lower IR. You can do this by the extremes and look at a 350mAh pack with an IR of 30mΩ/cell then compare to a 4200 2S1P of 3mΩ/cell. But there are more things than just capacity that can affect IR. Heat being a big one. You may not be heating a higher mAh pack as much as the lower mAh pack during discharge, which would lead the lower mAh pack to have a lower IR. I have some 4200 ULCG 2S1P packs that if you discharge at 35A will get hot (130*F), and they will have a nice low IR as well, then a 4400mAh 2S2P pack that barely gets to 110*F, IR is higher after the discharge than the 4200, but the discharge curve on the 4400 is a lot better, and if you take the IR when both batteries are at the same temp and the same voltage, the 4400 has a lower IR. So you have to be careful how you measure IR.

Another thing to consider, higher mAh generally means the battery can deliver more continuous current, if higher mAH batteries have higher IR, that means they would get hotter faster, which means they would be rated for less current.

Edit: to add and on topic to the OPs question. packs in parallel matter. I have 2S1P 5800 ULCG packs that have worse IR given same measuring variables than 4400 2S2P packs, from the same manufacturer. But this goes back to parallel packs having lower IR, not necessarily falling in the capacity argument. So again, lots of considerations...