Thanks all. The smuggery thing, as well as the hurt feelings, is what I was concerned about [wishes I could touch my toes/do cartwheels...]
The reason I thought maybe I should say something was that the DH's health seems to be suffering - much more lethargic and sleepy than he used to be, even six months ago. He's always been a bit overweight, but we spent a weekend with them a few weeks ago and I was really shocked at how unwell he seemed. But they don't see a problem, just say 'oh, we're going to get to the gym more'. But they don't.
I know my BFs mum has spoken to her about the weight and lifestyle thing. But this isn't 'you're looking a bit porky' it's more - I think your DH might actually be ill.
I know it is none of my business but to put it bluntly, I don't want to keep my mouth shut then see my BF widowed in her mid-forties, as her MIL was. And yes, they know they are overweight. But they seem to be in denial about the cause and the consequence: my BF will swear she doesn't over-eat, but when we were with her, she ate a cooked breakfast, two courses at lunchtime, cake mid-afternoon, three courses at dinner, and the best part of a couple of bottles of wine (plus peanuts, crisps etc).
And yes, I know this sounds like I'm watching, judging, perhaps being catty. But I'm genuinely concerned. Esp for her DH, given he lost his father at a very young age. And the fact he seems very unhealthy, whereas in their twenties, they were a bit overweight, but pretty fit and active.