Why is it considered a compliment for someone to tell you you've lost a "ton" of weight? All that communicates is that, at some point in the past, the person weighed more than a ton. What's wrong with "wow, you're looking terrific"? Specifics are not at all necessary in compliments.
And on that note, one shouldn't assume because a person goes to the gym every day that they are trying to lose weight. What if they're exactly where they want to be and trying to maintain?
Here's my advice for a pat response to "you look like you're losing weight" - -
"Really?!!! I'd better go see my doctor then. Golly, I hope I don't have cancer".
Then just walk away.