If your size limitations are based on the deck's abilities to hold up the weight, I would get a second opinion. There are many issues which will greatly change the weight of a tub, and one of them has just been mentioned: the height.
If your deck-checker is basing this size limitation on where the main beams of the deck are located, I would also get a second opinion: you can usually add a couple of two-by-eights or whatever size you need at the edges of the spa without a lot of effort. Depending on the design of your deck, you may not even need to set any additional posts to accomplish this.
I guess I just don't want to see you settle for less tub than you want for fear that bracing the deck is too big of a deal. It usually isn't much of a problem, but I haven't seen your deck.