I wouldn't worry about getting as big as a tank mate.
Work more towards getting your techniques spot on, then if you feel you want to get big, then do so.
However, bear in mind in muay thai, the bigger you are, the slower you're gonna be, especially if you go to fight in Thailand where they are lightning quick. I train in thailand and if i blink, i've missed a move, and im only 16!
They're so strong yet supple. The reason however, they seem to be able to defeat the bigger western lads is purely because their technique is 100%