Primaris Marines are indeed larger, however that's mostly to fix the proportions. Shoulder pads and heads can usually be freely swapped around and will match in terms of size. The difference is mostly in the legs, and a bit in the torso.
More important in your case, Primaris arms tend to be longer and a bit thicker. It can look okay - definitely from a distance - but will be noticeable up close, and the fit can be fiddly for two-handed weapons (due to the broader Primaris torso).
If you try and don't like the look of the normal Marine arms, some plasticard spacers to lengthen them mostly fix it IMO. You may want to experiment with something cheaper than FW bits to see if your inner perfectionist agrees though.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.