You are wrong again. When your 500,000 shares are at $3, then...

You are wrong again. When your 500,000 shares are at $3, then you are holding a far higher total value of these shares so of course a 20% of $1.5 million is greater than if the share price is only 50 cents and falls to 40 cents because your holding would only then move from $250,000 down to $200,000. Absolute amounts of $ changes is not the relevant thing you need to consider but the percentage amount.

20% is still a fifth of whatever it is that you personally hold. And 20% is 20% is 20% is 20%.........

And that is grade 4 arithmetic and logic......

As a proportion of a share portfolio, this 500,000 shares will grow if the share price of this share holding is growing at a fast rate than the rest of the shares in this portfolio. And, conversely, if this shareholding falls by a greater percentage than the prices of the shares in the remaining portfolio, then the proportion of the portfolio occupied by this share will also fall.