That is likely part of it and also explains why languages like Japanese are more tightly grouped, as there is less spread in word length for Japanese versus English or Italian.
Both of those languages LOVE to compound their nouns - smashing smaller words into massive ones. Like the simple "pasta + asciutta = pastasciutta = dried pasta" or not simple "Donaudampfschifffahrtsgesellschaftskapitän = Danube steamship transport company captain". All languages do it, but these do it with gusto.
I would guess, if it's solid empirical work behind this, that there's just greater differences internally between German and Italian speakers than for many other languages. Having lived in both Germany and Italy, I do not struggle to believe this is the case.