Applying Fractions (the Opentype feature) does not change the font style, that is, it doesn't change the text from Scala Sans to Scala Sans Bold. So if you see these characters get bolder and the font style does not change, then there is only one conclusion left: they are in fact drawn bolder inside the same font!
You can check this if you call up your Glyphs panel and select one of these bold digits.
(Of course this does not hold if, for example, you are applying the Fraction style using a character style which does define Bold as well. But that also should be clearly visible if you call up the Glyphs panel -- in that case, you would see the entire Bold font.)
Just googled the font and found this:
It shows the fractions are not bolder than the rest of the font. Of course, it depends if Jessica has bought it at FontShop or somewhere else. I suppose the font could be different from different vendors.
Thanks for your responses. When I imported the text, the fractions automatically showed up in a bold weight but the rest of the type was in a regular weight. I noticed when selecting the fractions to change them from bold to regular, the numbers become larger and the fraction is less refined. I think my big question is whether Scala is just drawn that way, or if there is something strange happening. I bought them from Fonts.com, but since all of the files on both Fonts.com and FontShop are .otf files, I figured they'd be the same.
I think I would first try uninstalling the entire family and reinstalling it. If that didn't work, I would contact Fonts.com and tell them your problem. You may have gotten a corrupted downloaded font and they should send you another copy.