The answer can be found at the end of Act II scene 1, when Mama tells Walter and Ruth what she has done with the majority of the insurance money. After she has told them she has bought a house for them all to live in, she asks Walter for her opinion about this act. Walter is of course very annoyed that she did not give him the money to invest in a liquor store, but note what he says:
What you need me to say you done right for? You the head of this family. You run our lives like you want to. It was your money and you did what you wanted with it. So what you need for me to say it was all right for? So you butchered up a dream of mine--you--who always talking 'bout your children's dreams...
It is this speech that leaves Mama "thinking heavily," and it is these words that make her reflect on her own role in the family. She comes to realise that she has not allowed her son to grow up and become a man because she has always kept the power in the family, and that if she wants him to become a man, he needs to have the power and responsibility to exercise in order to do that. This is why she makes the decision to give Walter the rest of the money.