All parts are slightly too small

I’m having an issue where everything that my MPCNC cuts seems to be slightly too small. This wasn’t really noticeable on very small parts, but larger objects around 10 inches or longer in each dimension are noticeably too short by about 1-2mm. Drilled holes are also slightly too close together; I’ve run multiple tests with drilling holes that are about 10 inches apart into foam and the holes are always about 2 mm closer than they should be. My machine has a mini-Rambo which was bought from the online store here, along with everything else except the printed parts. My belts are very tight, and I’ve also verified that the steps per millimeter value in the board’s firmware is set to 100. Should I need to change the steps per mm value, or is there another issue?

I would verify your bit diameter. If the stepps were wrong it would be linear not constant.