It's true that wheat was probably taller in the past, but I don't know how much I'd rely on that picture you posted.

One of the features of wheat domestication (and domestication of many other cereal plants) has been the introduction of dwarfing genes. This is because taller plants are more prone to lodging (falling over from the weight of the grain), more susceptible to wind damage. And also because taller plants spend more of their energy in building non-edible biomass (long stalks) rather than on producing more grain.

These days it's done by deliberately introducing and selecting for various dwarfing genes, such as Rht8 or photoperiod genes such as Ppd-D1. In the old days, it was more a hit or miss thing, but farmers have been selecting for shorter plants with more grain on them for a very long time.

However, I don't know that I'd jump to that conclusion from the picture you posted. It's not clear exactly what they're harvesting. It certainly can't be modern wheat, because it didn't exist at the time. Far as I know, ancient Egyptians planted einkorn and emmer wheat (and lots of barley for beer!), so a better comparison would be to the height of those plants rather than to modern wheat.

Also, I've seen plenty of paintings of ancient Egyptians harvesting shorter plants which also look cereal-like, so at least some shorter plants did exist at the time. I guess we'd need more information about exactly what crops these pictures show to speculate about any specific changes in plant heights.