In 19 DM, the spawn areas are very close to each other. Therefore, you can kill a player quickly after killing them. Therefore, you will get much more kills. In 20, 24, 25 DMs, the spawn areas are far from each other and weapons in the DMs are not good.Thats why you wont be able to get kills in these DMs as well as in 19 DM.Therefore, you will gain more exp in 19 DM compared to these DMs.