West Slavic Tribes in the 8th century

[Peterski]

Green colour represents sparsely populated / mainly forested areas:

Map is from book "Słowianie Zachodni" by Lech Leciejewicz (1989):

https://i.imgur.com/rkZQzxn.png

[SharpFork]

That's a lot of emptyness, what I don't particularly get is why is the pre-Carolongian border exactly where or even further west of where the Saxon Limes was, didn't the allied Slavs gain anything and even lost land in the war against Saxony?