While I agree basing can make a difference (especially as the smaller you get the bigger difference it makes proportionally, and it certainly didn't help with Mongoose Publishing's attempt at a modern warfare game, especially as the vehicles were already at a slightly smaller scale than the figures), I have actually compared the models with scale models of 1/72 scale. They tower over them… well, that may be an exaggeration. but they are far too big (while being too small to fit comfortably alongside 28mm models). Physical measuring of these models put them at being slightly too tall even if measuring to the eye, but they were much closer than other models tend to be (25mm from the bottom of the foot was probably about the bottom of the nose). Measuring 1/72 plastic soldiers (which are much more correctly proportioned, and visually fit perfectly with the 1/72 model kits) got measurement to the eyes of 20mm, but to the top of the head just about 24mm. So, if measured to the eyes, 1/72 seems to be 20mm, but if measured to the top of the head it does seem to be 25mm (or close enough to put them well within the margin of error).
Having discussed this I decided to investigate a bit online about figure scaling. Mostly similar things to what has been said ("Scales" are vary variable, different conventions of measurement, eye/top of head etc), but I think the thing that best sums it up is an entry on wikipedia's article on the subject. On a table of various miniaturing gaming scales it has an entry for "35mm". The description given for this scale: "Genuine 30mm".