You may want to check if maybe you don’t have too much tool deflection ? this can show up as slightly larger/smaller pieces than expected. I have covered it (not well enough) here, but first you could use @gmack calculator and figure out how much deflection you have for your setup/cutting parameters
Or if you don’t feel like learning about this worksheet just yet, just retry your cut with:
- the endmill stickout reduced to a minimum
- reduced depth per pass
- or even better, a roughing pass and then a separate finishing pass