Some — like the M — look pinched, while some are grossly expanded — such as the i or l.
Side bearings for narrow characters such as punctuation marks have to be puffed up.
So for the same reason that double-spacing typewritten lines is the norm, using two word spaces after periods is, too.
It’s a question of balancing the white space bound up in each character with the spaces around them.
Disruptive by modern standards, they were all the rage when this was originally set, back in 1774. The spacing used to justify these lines from Bembo’s 1495 work De Aetna is pretty irregular.
But the spaces between sentences are much bigger than a single word space, and they carry the brunt of the work of filling out the lines.
It’s the debate that refuses to die: Do you set one word space or two after a period?
In all my years of writing about type, it’s still the question I hear most often, and a search of the web will find threads galore on the subject.
In the early days of handset type, having the flexibility to exaggerate the spaces between sentences must have been a boon to quickly setting justified type.
And Aldus Manutius was famous as a thrifty, profit-conscious printer.