Mark don't take it to heart. Lexy, after her operations, and after the last one, can't jump high and being an indoor cat, no matter how many distractions I give her during the day she sleeps most of it... she's very sedetary and as such does not lose weight the way she should. She's just under 5kg... she should be 4 as like Willow she is a tiny cat....
She doesn't eat loads, she doesn't get more than she should... but she doesn't lose weight.
With 4 cats its tough to find a suitable solution for all of them, but I wouldn't take it to heart that she has put on weight. Its just one of those things, try a different diet if push comes to shove. Feed her in small portions and often if you can and see if that makes a change... maybe she will stop trying to nab the food off the others.