Toby talking about her overeating to Rebecca might also piss her off, and make Kate think he’s judging her.
This stressful situation is already a powder keg for drama, and Toby may have unintentionally set the fuse for a future explosion.
Weight has always been a touchy subject for Kate, which may be why Toby is keeping his gym time under wraps.
We don’t know how she’ll feel seeing him make such progress in the midst of this stressful time, though the typical Kate response to lies and deception is anger.