[tionate]

This is only true if those 500 shares had identical value, as market cap is the number of shares x the price.

[sobani]

They do have identical value.

500 shares of company A is worth 100% of the market cap of company A.

500 shares of company B is also worth 100% of the market cap of company B.

So if you have 5 shares of each, you'll have 1% of the market cap of each, even if one of those companies finds the cure for cancer or turns out to be a money furnace.