createrepo not attempting to create deltas

Hello. I do packaging rpms for game "UFO: Alien Invasion". I ran into an  
issue with createrepo.
New (bugfix) release of game has one additional file (all the others files  
werent changed). Package ufoai-data-server-2.5-1.noarch.rpm is about 350  
MB while new files are only 8 MB. So I decided deltarpm is a good way to  
conserve some hosting traffic and to save players some time. I made  
ufoai-data-server-2.5-2.noarch.rpm and fired up createrepo. At first it  
complained about size but I added option --max-delta-rpm-size 1000000000.  
Now it works fine except it doesn't even try to create deltas. No errors  
and no drms. What I am doing wrong?