It's not the size of the dog in the fight, it's the size of the fight in the dog...to an extent. The problem is having the right tools. Strength, stamina and ability. There are plenty of short/light people who can scrap, and big lads can get dropped with one shot.
I'm a former competitive combat sports athlete and we train with a lot of police. The issue they have is that the most time-tested, efficient and reliable methods for putting someone down and subduing them, they're not allowed to use until things have gone seriously south.
As we saw here, the officers took an absolute pasting, and still got a load of shit for it. Can you imagine the optics if one of them had choked one of these scumbags out?
Fair enough, they have to operate within the law under normal circumstances, but it's my opinion that they should still be taught those techniques as mandatory in order to have a better chance of keeping themselves and others safe in situations like this.
As for sending smaller people to deal with violent offenders, while they do have protocols for people known to be violent, or big/strong, or well trained, generally police aren't being sent in there with the aim of getting into a physical altercation. With sprays, tazers and strength in numbers, the vast majority of people can be dealt with by two/three officers regardless of size.
Your point is generally correct though. Even at my ripe old age I'd still back myself to do a fair amount of damage to the average untrained man, but the fact is, if I were to end up in a street fight with a 6'3 bloke, I'm very likely going to end up getting seriously hurt whatever the outcome.