Who holds patents for "over-size" golf club grips for putters, drivers, irons etc.,? Showed my design to a patent attorney, could not afford a patent search. Later, I saw my idea being sold! It was totally new! Nothing like it on the market at the time!
Performing a patent search is something you can do for yourself for free. This answer describes my process. Searching with the terms "oversize golf grip" on The Lens resulted in 428 granted US patents dating back to 1977. Please understand that obtaining a patent takes time, typically three or more years. If you saw a product that looked like your idea less than three years later, it is very possible that someone else thought of it first.
The majority of patents don't make it to the market, so the fact that you hadn't seen a similar product doesn't mean there wasn't already an existing patent. If you could link us to the product in question, we may be able to find out what the relevant patents are.