The reason RAM keeps getting brought up is because Terraria is not a normal kind of game. Most games only load up what they need at the time, whether that's a level, your inventory, or the creatures you're fighting. Terraria on mobile currently works this way as well by only loading the things around your character as needed.
PC Terraria (and now PS4 and XB1) doesn't do that. It loads ABSOLUTELY EVERYTHING when it starts up. Every item, enemy, creature, boss, and every single tile in your entire world map. All of it are sitting there in the games memory the entire time you're playing. It's the only reason the corruption spread works, because the game is updating every single part of the map at all times, even if you have never even been there. While a powerful processor is good, it just can't replicate what Terraria is already doing without the RAM to store that data. As 1.3 on mobile will be a port of the existing console code, the idea that it will mechanically work very similar the PC version is not an unreasonable estimate. Thus the amount of memory the game needs is probably going to jump up drastically.