My MZ boys had almost a 2.5 lb. difference at birth, and it's pretty much stayed the same since (though the difference is not so apparent now that they're 37 and 35 lbs). It's true, they didn't look a whole lot alike at birth (which in my opinion was a blessing). Now even I have a little trouble sometimes telling them apart in pictures (they have very different expressions/mannerisms in real time, so they look radically different to me and to people who know them well).
It's amazing to me how so many profess to not being able to tell them apart, when they look so different. Trust me, you will hear this as much as you hear they don't look alike. Even the twin moms I know that have b/g twins hear it.
If there's one thing that I've learned over almost 4 years of being a mom to multiples, it's that you cannot and must not stake your enjoyment, pride, or fuzzy feelings on anything people say--since you're likely to hear two extremes within 15 minutes of each other. People say dumb things, in person and in text, it's unavoidable. And your twins are (or are not) MZ by genetics, not from what some random stranger or auntie decides to declare.
I don't think they are likely to ever look the same *to you*. I have not met any mothers of MZ multiples that couldn't tell them apart near instantaneously on a regular basis. Nor have I met any mothers of more than one child who didn't occasionally have a clueless moment as to who they were addressing and getting them mixed up.
A 2 lb difference when they're 5 and 7 lbs is stunning. Once they get up over 20 lbs? Not so much. Don't worry about it.