I had a closer look at what the DS9 ruler was doing and how the cutout is being generated. The DS9 ruler measures the angular separation between endpoints of the region, I was looking at the size of the cutout in the image coordinate system. This is also how the cutout size is being generated, but in re-reading the SIA spec the intent of the SIZE values is to requests a cutout of a certain angular size. The difference here being the cos(dec) term.
So, the cutout tool has been changed to cutout a raster based on the angular size and should now return a cutout more or less the requested size at all declinations. Note however that cutouts may still be clipped to the image boundaries and so aren't zero-filled or anything to return a full raster. The SIA search only guarantees the requested position is covered, not the full cutout region, so this follows the "intersect=CENTER" model.
Thanks for pointing out the issue and please let us know if you still have problems.