Why is this genetive?

[dsimo04]

Line 3 of the preface to book one of Quintilian's Institutio Oratoria begins:

Quamvis autem non tam me vinceret praestandi quod exigebatur fiducia quam negandi verecundia, latius se tamen aperiente materia plus quam imponebatur oneris sponte suscepi,...

Why is onus in the genetive?

[adrianus]

Salve dsimo04

Personally, I believe the "oneris" qualifies "materia" and I would put a comma after "oneris".
Ego equidem hoc puto: oneris vocabulum vocabulo materiae adjicitur et commam post oneris vocabulum scribam.

Quamvis...latiùs se tamen aperiente materiâ plus quam imponebatur oneris, sponte suscepi
Despite...nevertheless the substance of the work being self-evidently more extensive than what was [being] proposed, I volunteered [voluntarily accepted]

[Alatius]

Hm, I would rather say that oneris goes with plus: "... sponte suscepi plus oneris quam imponebatur". I suppose you could translate this literally as "more of work".

The whole sentence then goes something like this:
"But even though I was not persuaded so much by confidence in fulfilling that which was demanded, as by the shame of refusing, nevertheless, as the subject unfolded, I voluntarily undertook more work than was imposed."

[adrianus]

I didn't think of that.
Id non consideravi.