Quick answer
Grasping produces conflict; trust produces sufficiency. The verse contrasts two ways of trying to get enough.
Read both lines
The Hebrew of the first line is רְחַב־נֶפֶשׁ, literally wide of nephesh, a big appetite rather than a haughty attitude, which is why versions render it variously as proud, greedy, or grasping. That reading makes the verse cohere: a man whose wanting is oversized keeps colliding with other people, and strife is the byproduct of an appetite rather than of a temperament.
The second line offers the alternative, and the verb behind "prospered" is יְדֻשָּׁן, made fat, an image of being well fed. Set against a wide appetite that never fills, the promise is not wealth but satisfaction, which is a pointed contrast rather than a payout.
What it does not mean
"Will be prospered" is not a financial guarantee, and the Hebrew image of being well fed is a poor foundation for one. The verse contrasts two dispositions and their characteristic outcomes, strife on one side and sufficiency on the other, and it is the same claim 15:16 makes when it prefers a little with the fear of the LORD to great treasure with trouble. Anyone whose trust has not produced money has not failed the verse.
Related: Proverbs 15:16 · Proverbs 16:8 · Proverbs 13:25 · Proverbs 28:22
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.