You need a reality check sorry mate.
These guys (AFL Footballers) are ELITE endurance athletes. Not 13 yo kids, but guys who train at an elite level twice a day, incorporating high altitude endurance work with searching lactic acid work.
There is NO WAY you get 17 in the beep test and there is equally (maybe even more so) NO WAY your 13 yo brother is beating 20-25 yo AFL footballers around the tan.
Furthermore, AFL players in the main, are far from 'bulky'...in fact most are almost lean muscle having body fat readings in the 5-8% range. They are cut but NOT bulky.
I am not trying to humiliate you, just amending your false claims.
AFL players are not elite endurance athletes (pro long distance track/cyclists etc. are). Their fitness training is based around being proficient in many general athletic abilities i.e. speed, endurance, strength etc. so the interplay between training different abilities (e.g. speed vs. strength vs. endurance) will tend to put some some sort of limit on how much they can improve them (e.g. as using more resources to improve speed, means less for other abilities like endurance) - so it's definitely plausible a young kid who trains for 3K could beat an AFL footballer, who is training to be more of an athletic 'all-rounder'.
Also, most AFL players would be ~10% if not higher. Most people's rough estimates of bodyfat are off the mark. ~5% is what you'd expect of a pro bodybuilder pre-competition. They do have low bodyfats but whether they are bulky or not depends on your reference point - compared to elite endurance athletes, yes, compared to pro soccer players, yes, compared to bodybuilders, no, compared to most pro rugby players, no etc.