While not explicitly documented anywhere, they automatically increase your storage limits once you approach a certain margin of remaining free space. That happens around Tuesday-Wednesday, they just add extra 5Gb to your limit.
I'm on the legacy free edition, and the auto increase worked for me as of November last year. I'm sitting on 41G used out of 51G limit, with photos taking up 29G. I have a second user in my workspace who also benefits from this feature.
Check out search results on "gsuite legacy storage increase 5gb", multiple users reported on their experience with it. It looks like extra space is granted if you're close to the limit, but not over it, for ~1 week.
There are also single reports on Google taking back the extra storage - back to 15(17)Gb - for people with extreme (ab)use of the feature, who stacked hundreds of gigabytes through 5Gb steps. Couldn't verify any of them.
I'm using 50-license GSuite since 2009, if that matters.