My DS is 3.6 and the fit as a flea. I took him to the scales yesterday at the chemist and his weight was 18.4kg (2st 12lb in old money...). He is approximately 100cm tall (3ft 4in).
He has always been a large boy, but there's not an ounce of fat in him, although he's never lost his little Budda tummy. He's very active (gosh, there's no keeping him still for two seconds!) and his diet is healthy, with lots of home-made nosh, fruit and plenty of veg.
According to the NHS BMI calculator, he is obese based on that diet. Ignore, fret like mad, or contact HV for a second opinion?