These before-and-after photos show why a 25-year-old trainer has a fitness app with more revenue than Under Armour's fitness app

Have you heard of Kayla Itsines?

The Instagram star and trainer has built a massive empire with over 5 million followers.

The 25-year-old Australian has become Instagram-famous with her Bikini Body Guides and her app, Sweat With Kayla, essentially puts her training in her fans' pockets.

Bloomberg Businessweek recently reported that her app has generated the most revenue out of all fitness apps in the past year - even big names in the industry, like Under Armour's MyFitnessPal's premiere membership and Nike's Nike+ app. (Though it's worth noting that Nike's app is free, while Itsines's is $19.99 a month. However, Bloomberg points to a $200 fitness band that some people purchase to use with Nike's app).

The star has cultivated an online community that calls itself "Kayla's Army," largely by sharing impressive progress photos - or "transformations" - of the women who are committed to her program. It serves as visible inspiration.

These are some of the photos Itsines has shared: