Kate also said her son, who is nine months old today, has been developing during the 19-day trip to New Zealand and Australia, which ends on Friday.
She spoke about George when she and husband William attended an afternoon tea party at an Aboriginal cultural centre in the shadow of Uluru, also known as Ayers Rock.
The royal visitors were joined by a group of leading students from schools and colleges across the region at the Uluru-Kata Tjuta Cultural Centre.
Some of them wanted to know about George, including Shannon Hunt, 17, who said: "We asked if they missed him. Kate said 'He's been changing so much while we've been away'. Kate also said he grew an extra fat roll while they're away.
"She also said he can get a bit grumpy on the long flights. She said she'd love to have him with her here."
George has appeared alert, happy and content whenever he has made an appearance with his parents and has looked noticeably larger since the tour began on April 7.
He has played a starring role at two engagements - visiting a playgroup in Wellington and a zoo in Sydney.
Before the royal couple left, Erin Keeley asked William if he and Kate lived at Buckingham Palace, and they shared her disappointment at visiting the royal residence last summer and not seeing the Queen.
The Duke teased her, saying: "That's because she knew you wanted a selfie. She ran away."