I'm not convinced it's backlash but it might be. Backlash on UM2 is much more rare.
Anyway, the feet are smaller for only one reason - shrinkage (possible 2: backlash) but circles are small for 3 reasons and so one expects them to be much smaller than desired.
2) STL doesn't support circles so instead you give it a bunch of lines but for some reason CAD software always puts the lines *inside* the circle and never outside. Don't know why. So the fewer the lines the larger the error. Yet if all your circles are 50 sided polygons that's just silly.
3) Rubber band effect. For PLA it has a long way to cool before it gets close to glass temp (unlike ABS). This starts happening the moment it comes out the nozzle so it is already cooling a bit and shrinking so the PLA comes out kind of like liquid rubber bands - pulling towards the nozzle which pulls in on the part and especially for inside circles, there's nothing to keep the PLA from pulling in but mostly air. On outer perimeters it's not a problem as the part keeps the "rubber band" from slipping in as much.
You have backlash issues. Notice how your circles aren't even close to being perfectly circular? And also how your infill isn't touching the perimeters properly? You probably need to tighten your short belts.
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