What is the reserved space for the snapshots?
By default, ONTAP reserves some space for the aggregate and volume snapshots. When an aggregat is created, 5% of the size of the aggregate is reserved for the snaphots. If a volume is created in this aggregate, 20% of the size of the volume is reserved for the snapshots. Therefore, if you want a specific size for your volume, you must increase the size by about 32% to get the size of the aggregate (if there is only one volume on your aggregate).