PDF and ePub formats are meant for different purposes. Each output format has its own rules for layout, etc. So, it can happen that the layouts do look different. PDF is meant to snapshot a document, almost as if you printed it. ePub, on the other hand, is a format that allows documents to flow and layout appropriately for different devices.
You may want to experiment a little with the layout in the original Buzzword document to make the pictures smaller, etc. and see if you still see different results between PDF and ePub output.