Below is the concept of snap delta. this will show the change in snapshot. it will not show the actual total size of snapshot.
snapdelta [ vol_name [ snapshot-name ] [ snapshot-name ]] Displays the rate of change of data between snapshots. When used without any arguments it displays the rate of change of data between snapshots for all volumes in the system, or all aggregates in the case of snapdelta-A. If a volume is specified, the rate of change of data is displayed for that particular volume. The query can be made more specific by specifying the beginning and ending snapshots to display the rate of change between them for a specific volume. If no ending snapshot is listed, the rate of change of data between the beginning snapshot and the Active File System is displayed.
The rate of change information is displayed in two tables. In the first table each row displays the differences between two successive snapshots. The first row displays the differences between the youngest snapshot in the volume and the Active File System. Each following row displays the differences between the next older snapshot and the previous snapshot, stepping through all of the snapshots in the volume until the information for the oldest snapshot is displayed. Each row displays the names of the two snapshots being compared, the amount of data that changed between them, how long the first snapshot listed has been in existence, and how fast the data changed between the two snapshots.
The second table shows the summarized rate of change for the volume between the oldest snapshot and the Active File System.