I think there were fat Elves. It is not just mentioned. And that's just normal. Imagine Tolkien describing for example the House of Elrond as : "there were lots of Elves. Some of them were fat."
You don't need to write about minor things like that.
Every elf not described as slender can as well be fat. We can't know. As well as every elf not described as right-handed might be left-handed as well as right-handed. It's simple.