The Milo Croton story is often considered to be an exaggeration (Guthrie, 1962) given that it invites the listener to imagine a man carrying a full-size bull across his shoulders.

In a modern breed it would be literally possible given that the birthweight of a male calf is around 30kg (Dhalek et al, 1996) although full-grown male of the kind we're led to imagine could be nearer 700kg (ibid.) and would surely be impossible to lift, even if it wanted to co-operate. We don't have any definitive data for the relative sizes of cattle in Ancient Greece but McInerney (2010) proposes that Greek cattle of the time were similar to steppe cattle with maximum bull weights of c.350kg, half that of today's largest common breeds.

In summary: most people could lift a bull depending on its age, it's impossible to know exactly what Milo Croto really lifted (if anything) or how far he carried it (if anywhere) but we can't say it's impossible given that the weight could be anywhere between 20ish kg and 700ish kg.

Sources:

Calf birth weight, gestation length, calving ease, and neonatal calf mortality in Holstein, Jersey, and crossbred cows in a pasture system; Dhakal K, Maltecca C, Cassady JP, Baloche G, Williams CM, Washburn SP; 1996

The Cattle of the Sun: Cows and Culture in the World of the Ancient Greeks; McInerney J; 2010

A History of Greek Philosophy; Guthrie WKC; 1962