It's just like cameras the better the lens the better sharpness and light gathering capabilities. A thermal is nothing more than a camera that sees heat but it still needs a lens to send that image to the core. This is just me speculating but I think IR Defense used a smaller lense with a better core to try and reduce prices and improve profit margin. The germanium lens is usually the most expensive part. I would love to see that BAE 12 core behind a 100mm lens.
With thermals there are many factors to determine magnification/detail, but the best/simplest way to compare thermals is to just go by the FOV and Resolution. The smaller pitch cores require a smaller lens for the same performance. You really are getting something for free here in terms of the size of lens needed for a given result as the manufacturers shrink the sensor size.
For the same reason, if you look at 320 vs. 640 cores in the same device using the same tech in the core, the 640 cores are necessarily larger than the 320's. That means you need a much bigger lens, just to get the SAME magnification on the 640 unit as a 320 one. Without a larger lens, in addition to the higher res core, you are only buying wider FOV.
Here is an example:
1.) IRH mk2 35mm 640 x480 12um 12 deg FOV (2.5x)
2.) FLIR RS64 35mm 640x480 17um 18 deg FOV (1.1x)
3.) FLIR RS32 35mm 320x240 17um 9 deg FOV (2.2x)
4.) FLIR RS64 60mm 640x480 17um 10 deg FOV (2.1x)
Edit to add:
5.)W1000-9 100mm 320x240 ??um 9 deg FOV (3x)
(I dont know this sensor pitch, but it's probably 30+)
The only advantage of #2 over #3 is wider FOV in 1x. When you hit 2x ezoom, you get the same specs as #3 (9 deg FOV (2.2x) @ 320x240). #4 is double the price of #3. Number #1 isnt.
A 12um BAE core device with a 100mm lens would be about 8x with a very narrow FOV. It would be awesome for longish range but almost useless at 50yds with running pigs and there would be no way to "zoom out" from that. A 60mm lens 12um core would give about the same magnification and FOV as your 100mm unit.
There are other things that come into play, but you can go by the FOV and Resolution and really should ignore the lens size. As you can see comparing #1 & #2, if you are looking at lens size and resolution only, you get very different performance.