Answer:
This simply means the possibility or extent one's life achievement depends on our personal self drive, struggle and determination.
Step-by-step explanation:
Your world is as big as you make it ;
Your world ; this represents one's personal life and all it encompasses ; the writer means that one's life achievement and the boundary of possibilities one can attain can only be a factor of one's personal input, desire, self determination, drive and mentality.
It can also be interpreted more simply as 'You are who you think you are', this also buttress the fact that the extent of one's achievement is a function of one's individual input and drive.