So, I reiterate, it is you who are a bit likely to have a big butt. Jupiter on the Ascendant does not guarantee that you will have a struggle with your weight, but that is a common challenge with Jupiter on the Ascendant.
We do not have your future wife's chart. Some astrologers might stretch your chart to suppose that your future wife might have a big butt because Sagittarius rules your Seventh and Jupiter, ruler of the Seventh, is conjunct Ascendant. Some astrologers might stretch the interpretation in that way.
I do not, however. It is simply common sense that your future wife might have a big butt, but Sagittarius ruling Seventh House is not a sure thing in that department.