I just realized something about Aurelion Sol's lore
The reason Sol doesn't just blow up everyone on summoner's rift in a single attack is because dealing that sort of damage to Runeterra would anger the Targonians and cause them to destroy another one of his stars.
It's not the he's weak in-game, he's just acting as though he's weak to avoid the consequences of using his full strength.
http://cdn.instructables.com/FT5/GF93/I70AWRQO/FT5GF93I70AWRQO.MEDIUM.gif