I don’t have the Gen 5, so I’ll let others opine. However, if you have the Gen 3, then the Gen 4-5 will feel smaller like the G21SF. When GLOCK went to Gen 4 it started with a 0.08”shorter frame (from backstrap to trigger) than the G21 Gen 3. The backstraps add 0.08” and the Gen 4-5 with the medium backstrap is the same size as the G21 Gen 3, with a larger backstrap, the Gen 4-5 would be 0.08” bigger than the G21 Gen 3.