As I understand it "mabibigat" is the plural form of mabigat. Co

Using the plural forms of adjectives is optional, so "Mabibigat ang mga libro ko" and "Mas mabigat ang mga libro niya" are both correct

Mabibigat is an adjective. It is usually used to describe plural nouns. For example, "Mabibigat ang dala kong libro" or "The book that I'm carrying are heavy."On the other hand, "Mas mabigat" is an adjective as well but it's used as a comparative adjective. So we can say, "Mas mabigat ang libro ni Ana kaysa sa libro ni Emma" or "Ana's books are heavier than Emma's books.

Those of us who have studied French or Spanish at school are used to agreement of gender and number which has to be rigidly and expressly stated in every single word which is capable of expressing gender and number no matter how redundant it may seem, but such things are optional in Tagalog. If you wish to specify that something is plural - which you don't have to do anyway - it's quite enough to do it once, although it is not wrong to do it more than once.
In "Mas mabigat ang mga libro niya" it has already been made clear that we are talking about more than one book, so no further clarification of this is felt to be necessary.