My 140 pound 6'1" brother is bowel incontinent and the legs & back waist of his pull-ups do not always hold in the feces. Does anyone have a suggestion for a better fitting pull-up?

You'd probably do best to go to online continence care retailers and research the very wide range of products they can offer. Many also have a customer helpline or live webchat which you might find useful for asking specific, personalized advice. As well as your brother's body shape, they may want to know more about his bowel habit too.

For my Husband I used to put on a pair of the "long leg" briefs (kinda like jockey shorts but not as loose) over the pull up or tab brief.
The underwear also kept the pull up or tab brief in place much better than if he wore just the incontinence underwear

you know those Lycra girdle things for women that come down just over the knee? Find one of those for men, or if impossible, try the xl or 1x for women.

The depends goes on first, followed by the Lycra garment to keep stuff in as it were. Changing himself would mean pulling the Lycra garment down first past the ankles, them doing the same for his pull-ups. The Lycra garment would have to be washed every day or max every other day obviously.

Another option is to medical tape the leg holes and then replace with every soiled pull-up.

If bro is not independent enough to do this, aides will have to toilet him, and if too expensive perhaps time to place him in a facility where others will see to his toileting and cleaning. Fecal incontinence is one of the main drivers for putting seniors into care, especially if dementia symptoms are showing up.

I don't think any of them are really good at dealing with feces, it just doesn't absorb the way urine does and overflow is a common complaint, and if the fit is are too snug they can be uncomfortable and even cause chafing and skin breakdown. When my mom had bouts of diarrhea wearing a pair of sports briefs (the kind with snug legs) over her pullup was helpful in containing things, that would still be laundry but perhaps not as much. Making an attempt to implement a toileting schedule where he makes sure to sit on the toilet after meals and every couple of hours throughout the day can be a strategy to limit the messes. Make sure he has pads placed on all of his furniture.