I believe you must be using a percentage (%)
of your current disk space before you can get an increase of space,
i think it must exceed 50 to 75% but your disc space is to low for an increase.
@spacresx is correct. You must be over 75% and there can be nothing violating the terms of service. Namely, you cannot have backups of your site or personal documents, you cannot store unrelated files or files greater than 10mb, and you cannot host personal files like a photo gallery.