Need Help! Another Newbie question

So first.
I get why everyone likes CNC routers.
I cut my first part this weekend and it was cool. My mind races with the things I would like to build that never could before building this machine.

But I ran into something odd, I snapped a bit while making my first attempt. I am guessing wrong type of bit or to fast not sure. Thats not the issue.
I attempted a restart after changing the bit. But I guess I didn't do it right so I messed it up. not a biggy, I knew going into this I would screw up more then I was successful at in the beginning.

so I started over. New bit all was good. Cut perfectly. With one small exception.
The whole part was 1/16th smaller on all dimensions then it should be.
I cut a second part that was drawn from this first part. Basically two pieces going back to back. But the one part is easily 1/16 smaller. I thought it was the bit and checked its dimensions. Thinking it to have slightly wider flukes.
Nope same diameter.

In the end its not a problem for what I built. but with no explanation I am concerned the next time it will be a bigger issue.

hmm..could a missed step or two do this.

I was cutting MDF with a 1/8th bit and mdf is mean stuff.

Just curious.