If file size is a concern, and you're willing to sacrifice quality or are unable to notice the difference, consider using a different switch. APS is designed to achieve transparent quality. r3mix was designed to make mp3s that sounded "perfect" to its' creator. Both with little regard for saving disk space.

--alt-preset standard -Y and --alt-preset fast standard both offer slightly lower bitrates in comparison to plain old --alt-preset standard. I've not done any comparitive tests, so I can't speak much about the resulting file sizes.

Or you can create mp3 files with an average bitrate (ABR), using the --alt-preset switch. Example:

--alt-preset 160

This will usually produce an mp3 file with similar/equal quality but uses less disk space than it's CBR counterpart.

That's because your making MP3 files with a variable, instead of a constant, bitrate.

That's just Windows Explorer/whatever improperly calculating the bitrate.

Yes.

Please do share how things turn out for you. Especially with ogg.