Here is an example of how a typical fight would go between a casual boxer and a casual grappler (in the case Brazilian Jiu Jitsu) practitioner. The boxer would get owned in most cases. They both seem at a comparable skill level of a a few years of training.
The one exception I have seen to this was Bas Rutten in his early days fighting in Pankrase. Before he became more well rounded he managed to beat quite a few wrestlers with his striking before being defeated by Ken Shamrock then after that defeat he upped his grappling game. Although even in the early days he had practiced some very minimal take down defenses and wrestling so he wasn't at zero but he was pretty crap at it but later became good after getting serious about grappling due being beaten by Ken Shamrock and another wrestler.Also in Pride FC a common thing the promotion would do when they wanted to ensure a win for someone they were trying to build up was to match him against some K-1 kickboxer that didn't know how to grapple.
To deliver a lot of force into a punch you need to be in a well balanced stable position with both feet on the ground and not leaning too far in any direction. Often when you are getting tackled you off balance and cannot get much weight into the punch meaning its just arm action without much body weight and the guy tackling you will just eat those punches and take you down. Mind you not 100% of the time but most of the time that is what happens.smash that grapplers face to a bloody pulp with 1000 Newtons of force as soon as he gets tackled.
One thing that helped Bas early on was that he fought in Pancrase where there was a gentlemen's rule that you didn't ground and pound your opponent and where there was rope escapes where your opponent had to release a submission hold if you grabbed the ropes. The fight also would get restarted standing which of course helped strikers a lotThe one exception I have seen to this was Bas Rutten in his early days fighting in Pankrase. Before he became more well rounded he managed to beat quite a few wrestlers with his striking before being defeated by Ken Shamrock then after that defeat he upped his grappling game. Although even in the early days he had practiced some very minimal take down defenses and wrestling so he wasn't at zero but he was pretty crap at it but later became good after getting serious about grappling due being beaten by Ken Shamrock and another wrestler.