Now lets not all jump on Pseudofire for what he said.
Not all goldfish keepers (even breeders) understand the importance of properly housing goldfish.
The issue with keeping goldfish in such a small tank is their waste. As you say, "they're goldfish!" You're correct, a lionhead is a fairly inactive type, when compared to a common or comet. However, a healthy fish should still be very active, no matter what type. And a healthy goldfish will produce a ton of waste. If you're overfiltering and performing large water changes often, they will still thrive in a tank that is just a little too small for them.
You're absolutely right, there's no need to buy a 60 gallon tank right away for a baby comet or a baby lionhead. You'd be spoiling the little thing. It would be overkill. A 20 gallon in that case, or even a well-filtered 10 gallon, would be fine. However, goldfish will certainly need larger tanks as they grow. The poster that suggested buying a 30 long for the lionhead is simply thinking of the proper size tank for an adult lionhead. And they're right- for a lionhead, a minimum of 30 gallons is correct for an adult.
Regarding your comment, a 10 gallon tank is much too small for four or five large goldfish, because of the sheer amount of waste they will produce. Your ammonia and nitrate readings will be off the charts. It's important to think about these factors, because although your fish might look healthy, their water parameters might be slowly killing them. Just as the air might look clear inside your garage, but you wouldn't think of using paint thinner in a small enclosed space, or leaving your car running for hours.
As a side note, you should understand that lionheads are a fancy type goldfish, and are no longer considered a coldwater species. Fancies do need a heater, as they are so far removed from their hardy coldwater common-type cousins. They're much more delicate.