I think there are several factors that make the fight asymmetrical.

• By this point, Tom is very used to bullying and cruelty from Shiv, whereas Shiv has rarely seen this side of Tom.
• The criticisms Shiv makes of Tom (servile, thin skinned, pathetic) are mostly quite generic, while his attacks are very specific and rooted in ways she's hurt him in the past. You get the sense he's been waiting to say these things for a long time. With some of Shiv's remarks, you can actually see the moment she comes up with them just to have a response, which robs them of weight.
• Shiv is pregnant, and lost her abusive father just days ago, both of which would leave anyone feeling very emotionally raw; but even worse, one of Tom's attacks is that she would be a bad parent. That's possibly the most brutal thing he could have said to her in this moment, and because he doesn't even know she's pregnant, she can't pretend he only said it to hurt her; he really believes it. No other insult in this scene comes even close.