How exactly is a 140 pound guy like Chauvin supposed to restrain a 6'5 265 pound man like George Floyd?

He did. He handcuffed him and had him face down on the pavement with 3 other cops to help him if he needed help.

It's probably less about using a knee on the neck to restrain an out-of-control person than it is keeping a knee on the neck of that person once hand cuffs are on.

Maybe ask the other three cops who were with him to help out instead of trying to showboat for the rookies. 

Sam here is an article a old one but it goes into detail as why police don't use 

tasers and I'm sure regulation limits where police can use pepper spray. 

https://www1.nyc.gov/assets/ccrb/downloads/pdf/pol...

1- Chauvin is 159 lb; he is heavier still with the uniform and belt on

2- Floyd was measured at 6'4" and 223 lb

3- Floyd was handcuffed

4- a one ounce bullet can kill an elephant, it is a matter of where it hits

5- you, yourself, are at least 2000 lb of concentrated manure

Chauvin used his gun, and then his handcuffs, and then his knee for over 9 minutes.  And during that 9 minutes he caused the death of Chauvin because after fully restraining him, Chauvin kept Floyd in a dangerous, oxygen-depriving position against all reason.

Taser, pepper spray, shoot the legs.