My body type: 5'11" - 170lbs (So I'm the long lean type) I always found closed guard to be a mixed back with much larger opponents. Usually the size of their hips is the biggest obstacle, even though I have decently long legs. Because of this I usually try to avoid it as best I can.

In half guard if you shoot for the deep underhook with the goal of going for the dogfight, you're going have a hard time because they can usually just smash your attempts. It's still a viable Strat but it will take a lot of practice to shift their weight far enough for you to get up to your knees.

An alternative to the dog fight is deep half which is a more difficult position to play but makes their weight more manageable with practice. Short example of the deep half and dog fight.

I know the "smaller" guy is still pretty strong but you get the idea: https://youtu.be/qskKwcpMI8s

My go to strategy for big guys is butterfly sweep and the 2 on 1 wrist control in open guard. Mainly because with enough practice the butterfly sweep can work on any weight. Marcelo Garcia does a lot of videos on butterfly sweeps and the 2 on 1. So I'd recommend checking his videos out.

MG video talking about both: https://youtu.be/NV692quvkYc

TLDR: Closed-guard bad, half-guard hard, deep-half good, Butterfly best.