Asset uses an internal cache for performance reasons, on your nas 512MB 30% to 80% is 256MB of memory, not a huge amount (considering entry level laptops come with 2GB of ram, which is 10x more ram).

Normally this cache is not used, unless something decides to index every tree that Asset provides, LG tvs are known to do this, other devices on the network could do this also. If you restart Asset and notice in the morning it is using lots of memory and you have not used Asset, then something else on the network is using Asset.