In the older fluff, Primarchs were not that big, they were just the size of regular marines.
Newer fluff supposedly reflects the beliefs of imperial people about their history, often exaggerated like a religious mythology. And here comes the retconning of most Primarchs\' features and background...
Most of the beliefs in the 40k universe are more myths that canon-real stuff. The shards of the Emperor\'s armor in the Crux Terminaturs, for example.
During centuries, people would travel the known world to venerate shards of what people thought were the “true cross of Christ.” But there was no way to prove the origin of these shards, and it is known that splinters were sold to believers for lots of money (enough splinters to build an entire chapel, it\'s said).
In the 40k universe, people don\'t understand the way force fields work, like crozius arcanums, rosarius and the crux terminatus, and they often prefer to think it is a miracle or divine intervention of some sort. It happened when Vandire tricked the first Sisters of Battle with a rosarius, anyway.
So objectively speaking, the helm of Russ or the helm of the Lion may not be and probably are not the actual real holy relics of the Primarchs, and are just reproductions of the original icons... Though say that in the Empire and you\'ll have the Inquisition behind your ass before you can say \"for the Emperor\".