The indo european "race"

Hello!
I know that "races" dont exist, and I know that indo-europeans may
have not been a unique race, but, how to explain the differences in
physical aspects for example between Italians and Germans? Already in
the Antiquity, Julius Caesar says the Germans are laughing at the
Latins because of their small size, and he often call them "giants"!
How can it be that, if, say, Germans and Latins were a unique people
for example in 2000 bc that they physical type so rapidally differ?
So, do you have explanations why it seems that Germans and Latins are
two populations very far from another genetically, though at the time
of Caesar they had been separated since only a few thousands years.

PS: do you know something about the hair colour of indo-europeans? in
the contrary to what I have said previously, many characters in latin
and greek litterature are blond!