Viber has introduced a new feature to reduce mobile data usage. The Restrict data usage feature is disabled by default but can be enabled in a couple of easy steps.
Open the Viber app and then tap on More > Settings > Media:
Tick the Restrict data usage checkbox:
NB: On Android, you can compare mobile data usage of Viber app before and after enabling this feature. Go to Apps > Settings > Wireless & Network > Data Usage and the select the Viber app from the list. Tap on the Data Usage cycle drop menu to view the data usage from the previous or current month.