I don't have any documentation to back this up, but as i understand it, this is what happens:

• If /usr/ucb/ps has no idea where it's output is going (ie: you are redirecting it, or on a dumb terminal) then it follows the strict definition in the manpage: 80 chars, unless you use -w in which case it's 132 chars, unless you use -ww(w) in which case it's as wide as possible.
• If you are using a non-dumb terminal, and /usr/ucb/ps can determine your terminal width, then it will use that width by default. if you use -w (or -ww or -www) it will output as much as possible

you probably aren't really seeing -w truncated to 80 characters when you pipe it -- you're probably seeing it truncated to 132, which is all -w is documented to give you.