Yes, they could reduce your percentage ownership of the company in the future, but this is normal and would generally happen to the other shareholders at the same time. For example, let's say you own 12%, but then the company raises money by selling 50% of itself to a VC. They'd do this by issuing new shares, doubling the number of shares in circulation, so while the total number of shares you held would stay the same, they would now represent just 6% of the company. Similarly, the CEO you mentioned would see his/her shareholding go down from 30.5% to 15.25%. This is what people are talking about when they refer to "dilution".
The important thing to remember with events like that is that they are -- or at least, should be -- good things. The money that the VC invested would go into the company, so the company's value would increase -- you'd have a smaller portion of a bigger pie. For example, if the company was worth $100,000 before the investment, your shareholding would be worth 12% of that, or $12,000. If the VC invested $150,000 for their 50% of the company, then the company would be worth $100,000 + $150,000 after the investment (just because it would have $150,000 cash in the bank), so your shareholding would be worth 6% of $250,000, which is $15,000. And, assuming the company is well-run, the money will be spent by the company on building a successful product, so it will increase the company's value above and beyond the amount of the investment and wind up making your shares worth even more.
(An aside -- I chose 50% because it's easier to do the maths -- a VC would normally take a smaller percentage.)
The one caveat to this is that shady boards of directors have been known to harm minority shareholders by selling newly-issued shares cheaply to their friends, diluting the other shareholders to benefit themselves. Your best defence against this is if you can trust the directors of the company; it also helps if there are independent (non-board) shareholders with large stakes and expensive lawyers -- one great advantage of having angel investors and VCs!
