If you join a larger company, yes you'll get less. If you stay in a company while it grows larger and you then negotiate for more options, maybe you'll get relatively many, as you're now rather uniquely valuable to that company. What people do instead is decide that options aren't worth it any more because how much the price will rise at this point, and ask for relatively small salary increases instead of relatively large option grants. This is doubly wrong since their salary is already pretty high at that point and there's a ceiling on salaries for various reasons that I don't fully understand but which certainly exist, while there's much less of a ceiling on option grants.

The percentage that matters varies depending on the stage of the company.

For a non-public company, you have to pay the strike price, and you get stock with a particular estimated value. But you have no easy way to convert that stock into money. This is worthwhile or not depending on the ratio between price and strike price, and depending on whether the eventual price matches the current estimate.

But if the company goes public, then the equation changes entirely. Now you can take out a short term loan for stock, buy at the strike price, sell at the market price, pay off the loan, and pocket the difference. Now you really should think of it as profit/option * # of options.

The longer that you intend to stay, the better your odds are that the second way of thinking about it is right, and the more valuable your options are likely to wind up being for you.

When did 100% of 5 become 10?

My thoughts exactly. Although, there are sometimes stock purchase plans that limit the raw number that you can buy (usually at a discount from the public price).