Deleting Apps Decreases Avaliable Phone Storage

Situation: WhatsApp used so far on iPhone5 (iOS 10.3.4) – need to use WA on newer iPhone SE (iOS 15.6.1). Backup of WA created – takes about 2.3 GB in iCloud. Now went to iPhone SE; has 32 GB of Space among was 28 GB used. WA tells me that's not enough space to restore backup and something over 1GB more is needed ... OK; I deleted a big bunch of apps (about 2 GB); tried again to restore WA backup, and it tells me that something more than 4 GB of more space is needed. OK; deleted another 3 GB of apps and tried again ... now WA is telling me again that it needs more than 1 GB of more space. But the most interesting thing is: iPhone Storage shows now over 30 GB used of 32 GB ... (that was less before); it is now "calculating..." for hours now. So as a whole: I freed up about 5 GB of space and have 2 GB less free space now than before??? What's wrong here??? Btw.: there are almost no pics, no docs and other space consuming stuff, the Space bar shows about 30% Apps; 20% iOS, a thin line for Photos, some more thin lines, and about 40% is greyed and "calculating" ...