Don't worry! According to my surgeon and what I've read, it's next to impossible to stretch your stomach. What has probably happened is you're fully healed now and swelling has gone down, so you're able to fit more in. Also remember that a tuna sandwich is pretty soft food, so we can eat more.
I remember at 3 months I had a really big tofu sub and I was surprised I could fit it all in! Just make sure you're taking small, slow bites (one per minute) and you'll be fine.