SD card free space keeps decreasing

Hello everyone,

I recently stated using Nextcloud and really enjoyed it.
My question is that I installed nextcloudpi on my raspberry pi 4 and store data on my external drives. However, I have noticed that the free space of my SD card keeps decreasing. It started from 53 GB on day 1 and now it is 42 GB.
Did this happen to anyone? Does anyone know what is going on?

Thanks

Can you share a bit more about your filesystem partitioning layout and mount points?