In some ways this is a product of the expansion of the wargame market and the popularity of 25mm, which of course was driven by Airfix plastic toy soldiers that were nominally 20mm, though actually averaging around 22mm.
I'm not going to rehash the old do we measure to the eyes versus to the top of the head arguments, because quite frankly they are ad hoc post justifications for the state of the hobby, and are just opinions about what is, or should be the standards that should be used.
Instead I will say that I think it is a reflection on the economic growth of the hobby and competition to produce the best selling range that one can, which is no bad thing.
Therefore I now want to point back to the title of this piece. I think that 6mm is becoming more popular, and the evidence is in the size creep that we have seen over the last 25 years or so, ever since Herioc's & Ros brought out their figure ranges in the then newish 6mm. They of course were less than 6mm tall, averaging around 5.5mm, and newer ranges increase the size and bulk of the figure, thereby producing nicer looking, and therefore better selling toy soldiers. Hurrah!
However, we are now at the stage where the market leaders are producing figures that are 8mm tall (sometimes taller) that are marketed under 6mm branding. Originally the Ral Partha BattleTech figures were to this Heroic 6mm size, though later on they were joined/replaced by true 6mm figures, becasue AFAICT there was a drive to be compatible with GHQ, who are steadfast standouts for 6mm figures being to a scale, and therefore approximately 6mm tall.
Recently though, Iron Wind Miniatures have started producing figures of battle armoured troops for BattleTech that are 10mm tor taller, but nominally falling into the 10/12mm size range. New standard infantry is being produce to match the larger figures, and so inevitably we have nominal 6mm figures that will be around 10mm.
For me this leaves me with one obvious choice. Go 10mm, as 10mm is clearly the new 6mm and get on the size creep bandwagon at the head of the curve, rather than be left behind with true 6mm. After all 10mm figures have more bulk, look better and paint up well.
So welcome to the world of Heroic 6mm, using 10mm toy soldiers to play games with.