Well you could use multiple PST files just for security. If everything is in 1 file, and that file gets broken/corrupt, you loose everything. I have a new PST file for every new year. Older files I hardly look in and it keeps my file size a bit lower.
However, with only a few hundred MBs, you will be safe with 1 file.
Don't know! Maybe hotmail estimates the storage size differently? Or standard Outlook has a better way of storing the data on the filesystem... no idea