I know that 'dense' can mean 'dumb', but it was clear from context that wasn't how I meant it.

I don't remember how the conversation lead up to it, but a girl told me how much she weighed. I remember being surprised because it was higher than I expected, so I said "Really? You must be dense!" and of course she got offended. But that's a compliment! That means she looks like she weighs less than she does. The opposite would be saying "Really? You take up a lot of space for your weight!" or "I thought you weighed more." which of course is rude.

Should I have found a better way of saying what I said/not said it at all? Am I the asshole?