"My cat is very heavy."

Translation: Con mèo của tôi rất nặng.

[JoeThompson23983]

Why is toi in this sentence?

[TehVanarch]

của + tôi = of/belonging to/possession of + I/me = my/mine. "My cat" is expressed by saying "[animal classifier] + cat + of/belonging/possession + I/me".

[Huy_Ngo]

What are you unclear about that?