You may be forgetting that the bigger donations (as amount of Bitcoin) were made when Bitcoin was small, most probably under 200$.
Many of the spent Bitcoin may be also at low prices. This means that if you calculate everything at today's price of Bitcoin you'll be awfully wrong.
Mind boggling that it took 20 posts for this to be mentioned.
These days if you're happy to donate 50 BTC to become an official donator then more power to you but you're likely to be in a microscopic minority. Once upon a time that was 50 bucks and less which is when many of the donations would've been made. The origins of this forum came together before Bitcoin itself even had a price.
True, the size of the donation is relative to the price, you cannot compare the 50btc donation when the price is at $100 and 1btc donation as for the price now. Don't just mind the amount if someone donates, instead thank him/her for the donation, no need to calculate how much is it in $$$ and I guess theymos isn't waiting for the price to spend those bitcoins, he spends it no matter how much the price is.