You may be able to reduce the PDF file size with careful selection of the export settings. For one thing, more than 150ppi for the images is probably wasted.
But PDF would again be the superior format, as it is almost universally accessible, easy to produce and update, and can be read on the web with partial/streaming download, making the overall size irrelevant.
All things considered, a book can be made about the same size in EPUB and PDF, so that aspect is unimportant. But an EPUB must be fully downloaded to read, and is highly dependent on the reader app or program used. Unless EPUB has some specific advantage for you or your audience, I suggest you stay with PDF optimized for web distribution and viewing.