If you grow the device (disk or partition) but not the filesystem, nothing special happens – the filesystem continues using only as much space as it previously did. For example, a 20 GB filesystem on a 100 GB device will only let you use the original 20 GB of capacity. (Not just any 20 GB total, but specifically the same 20 GB area that it originally occupied, i.e. nothing is written to the newly added 80 GB of space.)
This is what allows reducing the size – after shrinking a filesystem to a specific size you can be sure it'll only occupy the region starting with the beginning of the device, allowing the end to be chopped off.