How do you prevent yourself from being muscled out? They could easily take up the idea and implement it?
I wouldn't worry about the idea. I'd concentrate on implementation. User centered implementation can give you your edge.
Next, if it is truly a new idea and a good idea, the market needs to be developed. That is a horrendously expensive proposition. Why wouldn't you want well financed big companies to do that work for you? They're well adapted to bulldoze the market with brute force.
Finally, don't have one big idea. Have a lot of little ideas which fit together in a coherent whole or system. You can give away the "big idea" and make your money on three dozen interrelated refinements, improvements, and customer insights. They generalize, you niche. They zig, you zag.
Most ripoff artists can't understand or replicate a constellation of good ideas.
But aside from all that -- who cares? Let's say you have nothing ...zero ...zip ..nada. And through your idea, the big guy(s) make millions ...and you only made $300,000 on your first idea.
I'd reconsider feeling bad about that and have a whole lot more ideas for the bigcos to exploit.
The Myth of the Big Idea