The answer to your question isn't why are herbivores so big now? It's why are they so small? If anything, you think about the era of dinosaurs, you had mega animals who ate megafauna. Huge massive plants and nutrition there that fueled the growth of mega animals. Even the mammals of that time, and since then until relatively recent history, were massive. Giant rats compared to today. Giant sloths. Whatever we had today, there were giant versions before.

Humans, smaller than anyone else, just used the environment better. We burned it all down and killed the megafauna, and thus starved out the mega animals to become the apex there. So grass became the most common plant around, and those animals that thrived on grass stuck around. Those that didn't, struggled or went extinct. I think it's the book The Sixth Extinction which goes through this history very interestingly.

In the modern context, your question then comes down to how people massively underestimate veggies now. We've been taught that meat is why human brains evolved, why we grow muscles, and so on. Meat companies literally paid schools to put the food pyramid in with meat and dairy at the top and suggest it was the 'best' food. The top of the pyramid. And that marketing stuck with us and our assumptions about food.

But you take buffaloes, elephants, gorillas, giraffes, and other large land mammals and you see that the largest and the strongest are herbivores. If you think about what actually fuels a mammal's brain (glucose) then those question marks start to creep in. Protein wasn't the answer. Eating a lot more calories (and the particular nutrition) that fueled and allowed more complex thinking was.

Obviously, those animals can process grass. We cannot. We have to farm and grow different plants. But even for humans now, we get 2/3s of our protein on average from vegetables even with such a meat heavy diet today. It's just interesting that by historical standards, today's mammals are actually tiny compared to their predecessors.

What we can take from it for the modern world, though, is we shouldn't assume meat is the best nutrition. Your body needs nutrition. It doesn't really matter if it comes from an animal or a plant for that sake. You can be healthy eating meat or healthy eating a plant-based diet if either are well planned (both have respective risks and rewards). The biggest separator now, though, why humans are taller than ever, is more because total calories is up. The basic formula of calories in versus calories out explains most of this. Even for humans, we're taller than past generations because of this. We eat a lot more calories in childhood than we used to, so we grow more. As kids, that means growing taller. As adults, it means growing sideways.

But yeah, ELI5, is that we underestimate veggies. Even in terms of what you say about working out 5x per week. Meat isn't the crucial factor. Enough nutrients is (esp. in terms of enough nutrients absorbed). Quantity of protein is more important than quality of protein which is more important than timing of protein. You may also be comparing "normal results" to 'influencers' who are on steroids. So that's a whole other ball game. Almost all the popular fitness guys are on the juice. So "normal results" will differ massively there.