Where am I going wrong?

First I'd double-check your Body Media settings by using one of the TDEE calculators to see if it's at all off-whack or too sensitive to smaller movements.

Second, as others have said, it's VERY easy to eat portion sizes way beyond the MFP settings - I certainly did! A jacket potato can be 300g, chip shop chips 280g and a small portion of mash 180g, even if 100g is recorded. Cereal, pasta and rice are other common culprits.

Good luck!