In my experience, shooters who believe the .22 Magnum is more "explosive" on small varmints than the .17 HMR have often only used hollow-point HMR ammo, not plastic-tipped.
While lighter plastic-tip bullets in the .22 Magnum are more effective on small varmints than typical hollow-points, velocities in the low 2000's with 30-33 grain bullets do not splatter like 2550-2850 fps with 17-grain bullets. The difference is very similar to the .22 Hornet versus the .25-20, which the Hornet won long ago, due not just to more splatter but longer range with less wind-drift.
Once varmints get much larger than prairie dogs the .22 Magnum starts to show its stuff, but there are good .17 HMR loads for 10-pound varmints. As always, it's the bullet that does the job, not the cartridge.
