I'm thinking you don't have enough room to put your foot on the tread (front to back). To make that wider or bigger you will need to make a new staricase altogether.
Stairs have a riser and a tread, refered to as rise and run. To make the tread larger the stairs have to stick out in the room further. For instance, if you want a 1' wide tread and there are 12 steps, the stairs would have to have 12' of floor space from a plumb line from the terminus at the top to the bottom tread.
It is a major rework and seldom worth the trouble, expense and sometimes there is just not enough space left for it.
I meant to ask you if you have a nose or if there are just flush (see pic below).If there flush you can add 1-1/2" nose to them which does not sound like alot but it does make a difference, I did this for a client a few months back and she was very happy with the little extra I gave here.
Nice job Daryl. It does give additional space on the tread but, in some cases, it also presents a trip hazard with the 1 1/2" projection.
Very good point glenn, I walked up and down them several times and was not a problem I think mainly because they still were not the depth of a standard tread size. I staerted with 8-1/2" and ended up with 10" but still good point