I don't have any suitable 1/72 figures for comparison, but I can give you an indication of how their size compares against a number of different 28mm figures.
The first thing to note is that the size of the horses within the Thunderbolt Mountain range varies a lot. The first photograph shows some figures with the largest and smallest horses within the range. For reference the models shown are numbers 3002, 3012, 3013, and 3028. Most of the knights have horses midway between these extremes. The Green Knights horse is similar in size to the largest. This range of horse sizes is just about plausible, as at a rough guess, I estimate that the largest horses scale to around 17-18 hands high and the smallest 12 hands high.
After the Thunderbolt Mountain range went out of production I wanted to my collection, so tried using some of the smaller 28mm ranges. It sort of worked, but not really, so I am now duplicating many of the figures with 28mm models.
Size comparison photographs of some of the ranges are shown below:
The Thunderbolt Mountain mounted knights vary in height between 25 and 28mm tall, with the majority off the figures at the lower end of this range. Similarly the knights on foot are between 25 and 27 mm tall. The Green knight is approaching 30mm tall.
The foot figures are similar in height to the Wargames Foundry early medieval range, but like the knights are much closer to human proportions, so are much slimmer and have smaller heads.
Looking at the figure size for most medieval ranges, as shown on Plastic Soldier Review, I suspect that the Thunderbolt Mountain figures will on average be 2-3mm taller than most plastic ranges, but some of the horses will be significantly larger than the plastic versions.