This poster would be funny if it weren’t for the fact that the person who made it is an illiterate moron who doesn’t know the difference between “its” and “it’s.”
Incidentally, sentences and proper nouns are supposed to start with capital letters.
As to the OP, there’s really one big reason:
Money.
No, money doesn’t make you bigger, unless you eat a lot of it. However, it is worth noting that the rapid size increase of football players (and baseball players, and in fact basketball players and hockey players) has grown at exactly the same time that professional sports in North America has seen an enormous growth in revenue.
Pro sports have always been about the money, but since the late 1970s they’ve been about a lot more money than they used to be. Income tax cuts, higher salaries at the top end of the economic spectrum and more disposable income in the upper middle class have resulted in major increases in demand for big time sports. As a result, the revenues of the big sports leagues (and major NCAA conferences as well) have jumped upwards. So there’s way more money at stake than there used to be.
Correspondingly, all up and down the chain, from high school and junior sports to the pros, more emphasis is being put on scouting, training, and coaching. Say what you want about 'roids, but 30 years ago, baseball players didn’t even do serious weight training (or most of them didn’t; those who did, like Steve Carlton, were widely regarded as weirdos.) What training, nutrition and health knowledge there was in pro sports 30, 35 years ago would be considered medieval by today’s standards. Scouting is being emphasized more than it used to be; teams put more money into it because they have more money and stand to make more money from a successful draft, so there’s less chance now that a top offensive guard will be overlooked. Nutrition is better. Sports medicine is better. Teams now have a better chance to find and promote the big guys, to keep them healthy or return them to health if they are injured, and to increase their size as much as possible. All of this has happened, IMHO, because there’s just a lot more money at stake, so the resources available to find and develop the best talent are greater, and the motivation to do so is greater.
And yes, the steroids are better, but the truth is we’d see much bigger, stronger players even without steroids.