Well his wish was all about food, and because food in Terra was horrible he wanted to come back.
But MC become as strong as the g**'s because he spent to much time in Terra.
Think about: 1 day on Earth is 1 month in Terra. He was there for 7 Earth years that means that he lived in Terra for about 212 years. Time in which he was always fighting impossible enemy + the harsh environment.
Is normal to have become insanely strong....
And about traveling to worlds was one of the reward the goddess had given to him.