Your screenshots folder is limited to 250 MB by default. If you'd like to increase the size of your Screenshots Folder navigate to

This PC > Documents > Arma 3 - Other Profiles > Your%Profile%Name and open the Your%Profile%Name ARMA3PROFILE FILE using any text editor and add the following command without qoutation marks to the end of the file

"maxScreenShotFolderSizeMB = 2000; // 2 GB"

Then save the file.

Source for instructions: https://community.bistudio.com/wiki/screenshot