Why does my iPhoto masters file taking up a lot of storage?
The Masters Folder also contains some Pictures you already deleted. So you can empty the Trash (also a Comment to you Question).

This will clean up some storage. I had the problem that iPhoto created some duplicats, because i importet form my old Mac the iPhotoLib. If the Images on have other names but if the Data is the same you can find them with Tools like Gemini.

I hope this will solve your Problems ;)