I assume you’re dealing with raster images (.png, .jpg). A few tests I just made suggest that Luban imports .png images as 1px = 1mm, rather than respecting the ppi setting in the file, which would account for images getting grossly inflated in size. I would class this as a bug, since the unexpected result isn’t documented anywhere that I’m aware of.
(Method of test: I created two solid black square 64x64px .png images in GIMP, set one to have 144ppi resolution, and left the other one at the default resolution of 72ppi. Both were treated as 64mm squares by Luban. Two Luban versions were tested (3.7.0 and 3.15.1), so this is a long-standing bug. I would have expected to get one square of a bit more than 2cm and another half that size.)