Answer:
My backpack is heavier than yours.
Explanation:
Positive Degree simply describes people or things
Example: The atis tree is tall.
Comparative Degree compares two people or things.
Examples: The mango tree is taller than the atis tree.
Carlos is more obedient than Sophia.
Superlative degree compares three or more persons or things.
Examples: The acacia tree is the tallest of them all.
Carlos is the most intelligent boy in my class.
So we used heavier instead of heaviest because we compared two things.