Hi mdruiter,
you can define tab width both in "File Comparator" and in "Viewer".
File Comparator:
open File Comparator
go to menu Option - Configuration
see tab General - find text box "Tab size"
OR
from AS main window go to menu Option - Plugin Configuration - File Comparator
Viewer:
open Viewer
go to menu Option - Configuration...
in section General Option with text box "Tab Size"
OR
from AS main window go to menu Option - Configuration
in tree view navigation select Viewers and Editors - Internal viewer
find General Option with text box "Tab Size"
Both are by default set to width of 8 spaces. I found width of 4 spaces more suitable in my case
