Are you mixing capacity and weight? Never realized that you have 2 fields in material master and in the bin, one for weight and one for capacity?
Imagine if a bin has space for 4 pallets, then the bin is fully occupied when 4 pallets are stored in the bin, no matter how much product is on each pallet.
On the other side this bin might be in a high rack storage and the maximum of weight per bin is 1000 kg
Already a single pallet with a certain product could have a weight of 1000 kg, so it must be assured that no further material is added in this bin, even the capacity would allow it.
so you have 2 figures which work together to control what can go into a bin.