From the Super Mario Wiki
Mega Grrrols are large sub-species of Grrrols which first appeared in New Super Mario Bros. U along with their original counterparts. They are similar to Spiny Tromps from the Paper Mario series.
Mega Grrrols are found in a Rock-Candy Mines tower, Grinding-Stone Tower, where they and their original counterparts are the main obstacles. They have a similar role to that of the actual-sized Grrrols, except that they are three times larger.