Deleted thumbnail file but no space was freed
.thumbnails folder has a file of the format .thumbdata4* whose size is shown to be really big usually. But in reality, it's not a big file and it's size is shown as such is because it's not a normal file. It's a sparse file and it's real size can be seen using du command from a terminal emulator. ls shows the file size from it's metadata like most tools do and it'll be a big no.
On my phone, it looks like this
/storage/emulated/0/DCIM/.thumbnails $ du -h .thumbdata4--1967290299    
172K    .thumbdata4--1967290299 

/storage/emulated/0/DCIM/.thumbnails $ ls -alh .thumbdata4--1967290299                                                                  
-rw-rw---- 1 root sdcard_rw 11G 2020-07-30 19:18 .thumbdata4--1967290299

So it's actually a small file and you don't have to worry about it's size.
See the size difference. ls shows size to be 11GB while du shows it to be only 172KB.