I do not know if it is still the case, but if you used the "Camera Upload" feature in the past you could get extra space at 500 MB for each 500 MB uploaded up t

I do not know if it is still the case, but if you used the "Camera Upload" feature in the past you could get extra space at 500 MB for each 500 MB uploaded up to a certain capped amount. When the feature was in beta it was capped at 5GB extra, but before it went stable they lowered the cap to 3GB.