ZDoom

Don't know the exact numbers, but a direct hit on a boss takes like a quarter of their health on the third skill, so it is very possible a direct it from a nuke ends the fight instantly on easy.

How much damage is the nuke supposed to do, does anybody know by any chance? I mean, it's supposed to be easy on "Tiny Grasshopper", but I am really not sure if you are supposed to be able to one-hit that thing. Then again, if it's enough to bring it below 50% HP, it might be possible.

(In general, bosses are easy with Ripper Hearts, but that's a gameplay mechanic.)

Try it in DOS with the original executable and see. Start it on easy mode and then warp to level 4 and record how long it takes you to get the boss halfway. If it's different then the numbers may indeed need fine some tuning.

Well, I guess when you play on Easy, boss health is going down so fast it feels like a bug. It used to be harder in my memory, but my last playthrough was over a decade ago, so I just might remember it differently.

Looking at the code there's nothing wrong with it. The level end when the serpent has lost half its original health. The actual health value is skill dependent, either 1100, 2200 or 3800.
Behavior is exactly what's expected. The level ends when the serpent has lost half its health.

When fighting the Snake boss at the end of the fourth SW level, it is far too easy to end the fight. The cutscene already triggers when just doing a bit of damage, and if you attack with a nuke, the fight actually ends instantly.