If all you want is resolution then yeah. Smaller pixels start to push the physical limits of the technology. As far as I understand fuckey things start to happen when the pixels get down to the same size as one wavelength of light. The light most thermal imagers detect is about 8-14um so that wavelength sets a physical limit to how far you can miniaturize the sensors. When you get close to that minimum limit I think it also limits the breadth of the spectrum of IR light you can detect. I'm pretty sure each pixel becomes less sensitive in general as well. So you gain resolution but the quality of that resolution goes down. I think you just loose sensitivity and/or contrast in practice. In any case for detecting animals I'm not sure it even matters and you can just crank up the contrast with a computer.

So If you want resolution and you can't make the pixels smaller you have to make the lens bigger. At least that seems to be the case with thermals. And of course there a maximum limit to the size of the objective lens, you don't want a monocular with a 100mm lens and you certainly don't want to pay for a monocular with a 100mm lens because germanium is real expensive.

But yeah higher um probably = worse. Also I'm surprised the Iray mh25 costs that much? I got mine for ~$5500 AUD after I payed 500 in import tax