Translation:Your bag is heavier.
By the way, heavier is a comparative word, for me the sentence lacks somewhat to compare.
"Your bag is heavier than my bag"
Tas kamu lebih berat dari tas aku.
Why would "Your bag is more heavy" be incorrect?
Because of English rules.
More heavy doesn't exist, it's heavier.
Many short words apply that construction. (Eg. Happy/happier, naughty/naughtier, ...)