As I said before, this is true for the lower classes (peasants and merchants), but was not necessarily true for the nobility, who were all rich. They were two different worlds with two different visions of attractiveness. Look at the waistline of Aphrodite in the 1486 painting "the birth of Venus": she is not fat, nor skinny, but is instead very slightly curvy, so normal weight. Look also at the judgement of Paris (1528) by Lucas Cranach the elder to see yet more women of normal (if slightly skinny) weight.
As for men, most seem to be muscly or unfit at normal weight in Renaissance paintings. Look at Jesus in essentially every piece of Medieval art (the pinnacle of male medieval beauty) and you'll see that he is ALWAYS portrayed with a normal waistline, and never fat.
Yes, for the starving peasant a fat person would imply wealth, but beauty is
a). subjective (some people find fatness attractive, as others find old age, extreme height and extreme shortness also attractive) and
b). has not really changed since we were jumping around as apes. The muscly and fit are better survivors than the fat or anorexic and so they have always been viewed as more beautiful (don't even get me started on ancient Roman/Greek art).
So fat people were only as beautiful as 80 year old billionaires seem to young women from poor backgrounds today, as it often was associated with wealth.
Fat is a negative trait in the eyes of the opposite sex, and it harms health, fertility and martial ability, without a doubt.
None of this is true.