That is because the corresponding modern blocks often include, with the original 'insula', the space between it and the wall, and also the wall itself which has been disused and built over.
[59] See on this point some remarks by W.Barthel, _Bonner Jahrbuecher_, cxx.

101-108.
The public buildings of these towns are in general somewhat small and arranged with little attempt at processional or architectural splendour; they seldom dominate or even cross the scheme of streets.
Open spaces are rare; the Forum, which corresponds to the Greek Agora, contains, like that, a paved open court, but this court is almost as much enclosed as the cloister of a mediaeval church or the quadrangle of a mediaeval college.

Theatre and amphitheatre[60] might, no doubt, reach huge dimensions, but externally they were more often massive than ornamental and the amphitheatre often stood outside the city walls.

Here and there a triumphal arch spanned a road where it approached a town, and provided the only architectural vista to be seen in most of these Roman towns.
[60] In western Europe the provincial Roman amphitheatre averaged 45 x 70 yds.

for its arena.
Dimensions, of course, varied.