Shouldn't the URL for an arXiv submission point to the abstract rather than the PDF? This lets one read the abstract without downloading the whole document.
The abstract page (HTML plus all of the resources it loads) is larger than the full PDF.
But the size of both in bits is too small to matter. The real question is how long things take to render, and PDFs are slower.
Also, the abstract page has a clear link to the PDF, but most people don't know the PDF has a clickable link to the abstract page (along the left margin).
This paper basically suggests that any extraterrestrials are likely to be much larger than us, at around 300 kg, which is kind of scary...