Percent ownership is a vanity metric and is only useful for telling people you own some percent of a company. Ownership is subject to dilution (so the 0.01% of XYZ inc today may be 0.005% next round, etc.), and becomes functionally impossible to calculate how much that's worth (or project it into the future).
When it comes to equity, the numbers that matter are: the number of shares, the exercise price, and the sale price.
If I have 10,000 shares of XYZ, inc. with an exercise price of $0.10 and I sell them at $1.10, I've made 10k in gains [10,000 * ($1.10 - $0.10)]. Good luck being able to do any similar math when you're granted 0.01% of XYZ inc., since any calculation you do will go through "how many shares at what exercise and sale price".
Edit: since you work at Google, just imagine if they tried to give you 100 GSUs over four years as 0.0000002% of Google...
I agree percentage of the company is not fully representative of your compensation, but as a single metric it's better than the number of shares granted for non-public companies. Ideally, I'd like to know everything -- shares outstanding, shares authorized, cap table, strike price, liquidation preference, etc. But if I can only know one metric, I'd rather know percentage ownership -- from that I can make reasonable guesses about dilution, strike/sale price, etc.
For public companies who usually give RSUs (like Google) # of shares are better because you can easily compute expected compensation. Since they are RSUs, you also don't have to worry about strike price. Public companies also publicly disclose how many shares are outstanding. Those metrics are harder to find for private companies.
Sure, but the ceiling is meaningless without a sale price, so why even factor in the ceiling when you can do that based on what a reasonable price per share would be?
When it comes to equity, the numbers that matter are: the number of shares, the exercise price, and the sale price.
If I have 10,000 shares of XYZ, inc. with an exercise price of $0.10 and I sell them at $1.10, I've made 10k in gains [10,000 * ($1.10 - $0.10)]. Good luck being able to do any similar math when you're granted 0.01% of XYZ inc., since any calculation you do will go through "how many shares at what exercise and sale price".
Edit: since you work at Google, just imagine if they tried to give you 100 GSUs over four years as 0.0000002% of Google...