It’s important to note that magical spells can vary in strength depending on users ability and power.
“Avada Kedavra’s a curse that needs a powerful bit of magic be-
hind it — you could all get your wands out now and point them at
me and say the words, and I doubt I’d get so much as a nosebleed.
But that doesn’t matter. I’m not here to teach you how to do it.”
Fake Moody explains that the curses require strong intent, and magical power behind them to work effectively.
how Parvati Patil had produced such a good Reductor Curse that she had
reduced the table carrying all the Sneakoscopes to dust.
Here we see Parvati performing a Reductor Curse that managed to destroy a whole table. Harry, on the other hand, had trouble making a hole through some dirt with a Reductor curse in the maze in Goblet of Fire.
Even earlier then this we see Lupin teaching Harry the Patronus charm.
“With an incantation, which will work only if you are concen-
trating, with all your might, on a single, very happy memory.”
said Harry, quickly forcing his thoughts back
to that first broom ride. “Expecto patrono — no, patronum —
sorry — expecto patronum, expecto patronum —”
Something whooshed suddenly out of the end of his wand; it
looked like a wisp of silvery gas.
“Did you see that?” said Harry excitedly.
Here we learn that the strength of the happy memory is what increases the power of a Patronus charm. We see that while a semi-happy memory creates a misty low powered patrons, with a very happy memory the spell works at full power and creates a corporeal Patronus.
Hatred rose in Harry such as he had never known before. He flung
himself out from behind the fountain and bellowed “Crucio!” Bellatrix
screamed. The spell had knocked her off her feet, but she did not
writhe and shriek with pain as Neville had.
“Never used an Unforgivable Curse before, have you, boy?” she yelled.
She had abandoned her baby voice now. “You need to mean them, Potter!
You need to really want to cause pain — to enjoy it — righteous anger
won’t hurt me for long — I’ll show you how it is done, shall I? I’ll
give you a lesson —”
Bellatrix explains that you have to MEAN it for the curse to work
effectively. Hermione and Ron’s confused queries about Travers and
Bogrod, who were both now standing there looking blank. “I don’t think
I did it strongly enough, I don’t know. . . .” And another memory
darted through his mind, of the real Bel- latrix Lestrange shrieking
at him when he had first tried to use an Unforgivable Curse: “You
need to mean them, Potter!”
So now to circle back to Snape, he is an extremely accomplished wizard, one of the most powerful in the entire series behind Voldemort and Dumbledore. He both has the skill and power to amplify a basic curse to add extra force behind it. As we see just one year later, three disarming spells from Harry, Ron and Hermione have the same force behind them as Snape.
“Expelliarmus!” he yelled — except that his wasn’t the only voice
that shouted. There was a blast that made the door rattle on its
hinges; Snape was lifted off his feet and slammed into the wall,
Finally Snape intentionally uses far to much power on his Expelliarmus spell then needed to embarrass Lockhart.