What kind of underwear briefs should I buy for my mom if she has leakage? She is only 85 pounds and I’m assuming the norm might be too big.

Depends and Silouette both have extra small. I used XS for my aunt. She was very petite -

I have found the Because Market overnight maximum briefs in small fit my mom who is about 80 lbs well, and are well made and comfortable. We add a booster pad at night to the brief. They also have a Premium Brief for the daytime. All are well designed. You can set up automatic shipments which is also great.

Someone once suggested to add an extra pad with the underside slashed open underneath if you want to try it

There are some pull up products for children. Just did a quick search and one of them starts at 85 pounds up to 140 pounds (this was 1 product and the XL so the smaller ones might even be better for your mom)
So begin looking at some of the larger "child" size products and many are intended for "over night" use so they would be pretty absorbent.

When we had a bed wetting child there were no child sized pull ups big enough, you could try but I doubt they will have anything to fit.
And I found having a very snug fit around the legs was a two edged sword in that coupled with moisture it caused skin break down, I figured occasional leaking was better. Have you tried different brands?

I'm coming back because it just occurred to me that you might be talking about urinary leakage rather than leaking pull ups that aren't doing their job.... if that's the case then I would try using pads made for urinary leakage, they make ones that are good for light dribbles all the way up to what is essentially as large and absorbent as most pull ups.

I think Someone posted ages ago to try a baby pull up. The biggest you can get.