Even though I'm not a girl, I'm a guy, I'm gonna tell it like it is. (Because I haven't seen this thorough of an explanation.)
Why size matters, but why ultimately, it’s knowledge. (Also not the ‘motion of the ocean.’)
First why size does matter.
Because most guys don’t know what they’re doing, and generally just blindly thrust straight in out of ignorance, a larger penis is able to better fill a vagina, and therefore is more likely to hit the sensitive areas simply by happenstance. But even then, it’ll be inconsistent without the proper aim. But an average to smaller penis is far less likely to hit the sweet spot without the knowledge of where to aim it, or the luck certain positions may provide.
For those of us who are average (like me), or smaller, size matters for us, in that due to not being hung like a horse, we’re more likely to seek the knowledge we need to satisfy our woman and overcome our perceived short comings. Now that could be learning how to work her clit (though that’s best to warm her up, but better than nothing), finger her (which also requires the knowledge of where to go, but if you try rubbing everywhere, you will eventually find it; also, it does allow you to find it in her first before your penis has its turn, which is a handy trick to use with a new girl, sending your fingers in on a recon mission before sending in the heavy artillery to finish the job), or how to aim what you have (like I have). Or, you can be lazy, and just let her ride you, because she’ll make damn sure she rides you so that you hit her sweet spot. (And if you pay attention, you can feel what she’s pressing against you, so you know what to press against when it’s your turn.)
Now why knowledge matters, and not size. (Nor the 'motion of the ocean.')
You have to understand that like our penis, stimulation to certain areas of the vagina feels better than others. The best places on a penis are the head and underside, and rubbing the rest of it isn’t really the same. It’s true for women too. While just a thrusting with no real purpose might feel okay, if you don’t hit the spots that feel the best with purpose, it won’t lead to much, and certainly not orgasm without size or luck on your side. The head of our cocks are basically their clits, and as for the underside of our dicks, there’s a reason there’s that gap between their legs, just under their bottom. We’re supposed to take them from behind, rubbing our sensitive underside against their sensitive area too, so we’ll press harder against it.
The sweet spot isn’t very hard to find if you know where to go, nor is it very deep, and is in the same basic spot on all women. (Which is why it does help to send in your fingers on a recon mission to properly scout the area on a new woman.) If you’ve ever seen a woman finger herself (palm on her pubic mound and fingers curled inside herself, putting her sweet spot at her finger tips), she only gets two joints of usually two fingers into herself, proving both it’s not deep, nor does she particularly need to be filled to get off. If you’re hung atleast as well as your thumb, you’ll be fine.
Once you’ve learned where to go, you wanna work her clit to warm her up and get her non-proverbial juices flowing. Then finger her to atleast one, better two, orgasms as you feel for her sweet spot (she’ll let you know by how she reacts to the rubbing) before you get your fun, making it far more likely that she’ll cum on your cock, because you’ve primed her orgasmic pump. That way you have atleast ensured she got some pleasure no matter how long you go, and your thrusting will feel good to her.
Now as far as the ‘motion of the ocean,’ there’s no special pattern or technique that you need, just correct aim and a steady pace that you can maintain long enough for her to reach climax, increasing the speed and force near her orgasm to make her cum better. And if you’ve fingered her to atleast one, or better two, she’ll be more sensitive from her previous orgasms, like we are after ours, but it’ll make her cum easier and easier.