Typically it's .jpg files that are larger, especially when taken by 20+ megapixel cameras.
Why does there need to be a different size setting for different attachment types? The main reason for having a size limit is to help manage server space so that it's not filled up quickly with lots of huge files. To that end, it's irrelevant what file type it is, a 20Mb file is a 20Mb file.
I think you may be confusing measurements for file size.
I've never had .pdf files as big as 15Mb, most are just a few hundred Kb in size. As for .jpg files, you generally can't get a decent picture size at less than 100Kb, sizes smaller than this are generally suitable for avatars.
Anyway, there isn't an add-on currently available for this as far as I'm aware. I don't think this would be a popular add-on for the reasons I've stated before so you'll likely need to pay for custom development. Good luck with that.