Like with most things in life getting things in proportion and to the right scale is important, it is no different when it comes to framing art work.
For example if you were to see a giant riding a tiny bike that would look funny to many as it just isn’t in scale, the same will go for art work if they aren’t in correctly proportioned frames.
The types of scaling for frames can be determined by the art work itself. When you have a piece of work that is strong, very intense full of colour and bold images then a very striking frame with thicker edges will be needed so that the frame doesn’t get lost in the painting. If you have a very delicate piece of artwork, with soft shapes and colours then a more plain and thinner frame will be needed so that the frame doesn’t take the focus away from the art work.