I summarized relevant sources before in: The Viking leader Rollo the Walker (Göngu-Hrólfur) was supposedly called that because he was too big to ride a horse. How large would this have made him, considering the horses he would have had access to?

+++

In short answer, History of Norway (Historia Norwegie), one of the oldest Scandinavian historical writings (though in Latin) customarily dated to the middle of the 12th century, associates the famous moniker with Rollo of Normandy.

BTW, do you remember any concrete article/ book stating historical Rollo was really about 140kg? That book might have been very helpful for me to confirm some basic points when I had to check the relevant information (like the plausible carrying weight of the Icelandic horse in pre-modern period......).