Maximum file size for imported videos and .story file?

New to Storyline - I usually export my produced videos out of Adobe Premiere at Vimeo settings 1280 x 720. These have proven to be optimal data size and data rate for web. Our story size set up in Storyline is 720x405 (16:9). I scale the video size down once imported into Storyline. I'm under the impression that choosing no compression will preserve the quality of the 1280 x 720. The videos seem to be loosing quality upon publish even though I've chosen no compression. 

Is the mismatch in size the problem. I would expect that if I were going in the opposite direction, but I'm putting a higher res video into a lower res storyline project.