Conversion and storage of glucose to glycogen
Glycogen storage limitation is due to these additional facts also: 


Glycogen is a hydrophilic molecule. This means an increase in glycogen concentration will cause a proportionate increase cell weight due to water accumulation. As cells can contain only limited volume the amount of glycogen stores are limited. Lipids are hydrophobic hence can be accumulated without increasing the volume and weight of cell due to water accumulation. 
As a source of energy lipids are approximately 2.5 times more efficient than glucose.