iCloud Drive Storage

I have downloaded iCloud Drive onto my Windows PC. I want to use it to back up my computer files and photos. I copied lots of files across fine into the folder, then it was full. I have paid to upgrade to the 50mb (0.79p month) to gain more space. However, despite showing up that I now have 50 mb of storage on my phone and the iCloud app on my computer; when I try to copy more files into my iCloud Drive, it says I do not have enough space. There is no way the files I am copying are filling 45 mb! What am I doing wrong? Why is does my iCloud Drive now not have space for more files?

Thanks for your help.