Just some thoughts and some of you may think this is nuts but it could be the factor.
You said you train abs and hard all the time. Are you doing a lot of weighted ab exercises? IMO, weighted ab exercises are not something that need to be done in most cases. Added weight to ab exercises will cause your ab muscles to be bigger...it's not rocket science.
Do you do heavy deads, and squats? This will add to abdominal growth. You can throw heavy BB military in that same category. If you are doing those things, and training abs with weight, well it doesn't take a genius to figure out why your abs are so big.
Dead lifts: if you are doing these, start doing them from the rack instead of the floor. Deads from the floor can be a great exercise but in many guys they can cause the midsection to look a lot wider and thicker. Do them from the rack and you still get the same back benefits. No, you won't get the hamstring benefits but no big deal, worry about your hamstrings when you train hamstrings.