The answer to both of these questions is, of course, never. Never, ever, ever. Not if you say it in a loving way; not if you say it in the spirit of constructive criticism; not if actually the exact opposite has happened and you’re just being ironic; not if you’ve just taken her shopping on Bruton Street and you think you have earned yourself a Get out of Jail Free card; not even if she has canvassed your opinion and has specifically asked you to be totally honest. Not even then. In fact, especially not then.

The reason a man should never say these things is that a woman will take it badly. Trust me, I know. I’ve been living with one of them — a beautifully slender, not remotely porky one, obviously — for nearly 15 years, and never once have I been able to discuss her body shape in even the vaguest and most delicate terms without landing myself in the deepest doo-doo.

I thought I might get away with it with my five-year-old daughter, but even that’s not allowed. “Hello, thunder thighs,” I said to my chunky little Poppy the other day. “Don’t you dare call her that,” said my wife. “I used to feel crucified by remarks like that when I was a child. You’ll give her a complex.”

I think she may have a point. When women behave in a bizarre and irrational way that men don’t understand, it’s often the case, I’ve found, that there’s an underlying biological or sociological reason. Chicks biting your head off when they’ve got PMT, for example, is nature’s way of suggesting that you shag them at a different stage of ovulation. And the reason, I suggest, that women are so hypersensitive on the subject of weight is that it’s a brutal reminder of what utterly shallow bastards men really are.

Of course, after decades of postfeminist indoctrination, men aren’t meant to be this way. We’re supposed to have learnt that looks aren’t nearly as important as character and a lovely sense of humour; that cellulite is the norm and not some hideous excrescence akin to leprosy; that the right weight for a woman is the one that makes her happiest. Unfortunately, while our brains may have been persuaded to think in this enlightened way, our penises have not. And it’s our penises, unfortunately, that make many of our really important decisions. Like when to stay faithful. And when to chuck her for a younger, slimmer model.

This is what makes women so cross and upset. They know on an instinctive level that no matter how wondrous their inner beauty, their lot as females is to spend their lives fighting a constant battle to stay within their males’ threshold of minimum acceptable attractiveness, or risk being tossed carelessly aside. So while, for the male observer, getting het up about your weight may seem just another of those infuriating things that girls do, for a woman it’s more a case of clear-eyed realism.

The problem with us men is that we’re generally very conservative. When we start going out with a woman, we do so in the expectation that she is going to remain more or less as she was when we bought into the package. Obviously, we’ll make allowances for things like the ageing process or the effects of childbirth. But anything too radical — a dramatic haircut, a brand-new style direction, or excessive weight gain — and we start to worry. It feels like a breach of contract.

This instinctive conservatism fears change in any form. Certainly, the times in my marriage when it has come closest to disaster have been those awful moments when my wife has suddenly announced: “Right. Things have got to change round here,” and started trying to make me do more childcare, or more laundry, or less reading of the newspaper during meals. “What?” I want to reply. “This is me. This is what I’m like. If you want to live with someone who’s not me, go and find someone else.” Luckily, it never comes to that. After a few token gestures, I’m off the hook and things soon settle back down to their natural state.

I’m sure this is how a lot of long-term relationships break down, though: either when the woman suddenly tries turning her man into something he’s never going to be; or when she gets bored with her lot and decides to reinvent herself, and her man goes: “Hang on, this was never the deal.”

And there’s a jolly good reason for this male yearning for stability. As a chap, your job is constantly to look at the bigger picture — not just your woman (still less tiny nuances in the way she looks: eg, the haircut she told you she was getting this morning that you still haven’t noticed), but also your home, your job, your kids, your ambition, your money, your health — and if everything is to tick over smoothly, you can’t afford to have too many inconstants. Life is unreliable enough without your wife or girlfriend pulling weird new tricks on you. You need her to be there for you, and looking like she’s supposed to.

So long as women understand this basic truth about their men, I honestly don’t think they’ll have much of a problem with them.

Yes, girls, you’re quite right — it really is the height of impertinence for your man to comment, even in the tenderest terms, on the size of your thighs or buttocks. Just so long as you understand that it’s your responsibility to keep them in check.

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