The 47-year-old actress won't take credit for her fiance's polished new image and says he slimmed down of his own accord.
She is also adamant the Australian cricketer hasn't had any cosmetic surgery to improve his looks and is completely ''au naturel''.
Speaking on British TV show 'Lorraine', Liz admitted: ''I love him looking like that, I think he looks beautiful. He retired from international cricket before I met him and I think even when I met him two years ago he was still much bigger because he'd still been training.
''Even though he still does a little bit of cricket he doesn't do as much. But as far as being slimmer, I have to say it's entirely him.
''He's never had any surgery and is not on a diet and he doesn't wear make-up - look at those eyelashes, they're very curly.
''For the record, he is au naturel. No assistance, especially not from me.''
The 'Gossip Girl' star also revealed she was shocked to see the recent photographs of Lady GaGa wearing her iconic Versace gold safety pin dress that propelled her into the spotlight at a 'Four Weddings and a Funeral' premiere, which she attended with ex-lover Hugh Grant, in 2004.
Liz insists the pop superstar did the dress justice and she thought Gaga looked incredible.
She gushed: ''I thought she looked fantastic in it. Really gorgeous. It was a real surprise, somebody actually tweeted it to me, 'Have a look at this!'
''I thought, 'Good heavens!' I had missed the news for a few days because I was in America. I thought she looked gorgeous.
''I know it went to some Gianni Versace museum and was snatched away from me 18 years ago but I have a feeling it was exactly the same one.''