Only rule of thumb everyone should follow is don't invest what you can't afford to lose. If you fail to do that and your assets went downhill, you're properly fucked. That being said it is true that the biggest winners are the ones who invest the biggest bags. But those who really win here are the ones that have the balls of steel and would invest their money, wait until the market is much more favorable, and then take the winnings, those are the ones that deserve my salute.
I think people who have a lot of fund will have opportunity to gain bigger profit. But it's not a guarantee. In my opinion, people who can gain profit from this investment are those who buy and sell in correct time. I only have small amount of money but it always doubled every bullish market trend. So I sold at that time then buy btc again when bearish market trend happen. Another example is, I bought TRX at lowest level at bearish market few weeks ago and now if I sell it then I can get 200% of profit, but I will not sell it now because I think bullish market trend is even not starting yet. So in my point of view, investing in bitcoin and other cryptocurrencies need knowledge and experience, not just big fund.
Long as the guy who invests have prior knowledge about investing in the first place, he should be good to go. The thing about having much more money is that you can diversify your assets, this will minimize your chances of overall losses and therefore will allow you to win more. The poor and the ones who are less fortunate do not have this luxury so we see small-time investors losing all of their stuff in one go.