Isn't the point of alchemy is that you can only get as much as you give?
Yes it is. The law of Equivalent Exchange states something like: "In order to obtain or create something, something of equal value must be lost or destroyed."
According to the FMA wiki:
In standard practice, Equivalent Exchange is separated into two parts:
The Law of Conservation of Mass, which states that energy and matter can neither be created from nothing nor destroyed to the point of elemental nonexistence. In other words, to create an object weighing one kilogram, at least one kilogram of material is necessary and destroying an object weighing one kilogram would reduce it to a set of parts, the sum of which would weigh one kilogram.
The Law of Natural Providence, which states that an object or material made of a particular substance or element can only be transmuted into another object with the same basic makeup and properties of that initial material. In other words, an object or material made mostly of water can only be transmuted into another object with the attributes of water.
Source - Alchemy - Equivalent_Exchange
This can be better understood when
Ed repairs Al. Al has a huge hole and Ed cannot create 'new armor', so he has to stretch the 'available' armor in order to cover the hole. However, as it stretches, it becomes thinner, since he cannot create new material. He also uses some pieces they were able to retrieve, and those he can fuse together into the armor again, using the same mass. However, as explained above, missing pieces cannot be created out of nothing, thus the need of the stretching part.
Although I am not quite sure about the last paragraph (concerning the Law of Natural Providence), for it makes me think about how the philosopher's stone really works...
How does that work?
The philosopher's stone
is a concentration of human souls. Thus, it uses those human souls as the 'giving' part, so it appears that you are 'getting' something without having to pay for it. However, it was already payed for. So it creates the illusion that you are bypassing the Law of Equivalent Exchange.
It should be taken in consideration that this means the philosopher's stone is not unlimited, since the human souls contained in it will eventually be consumed.
However, I have some doubts as to how it works exactly, considering the Law of Natural Providence explained above, because you may use human souls to transmute... just about anything. As discussed with Madara in the chat, the human soul is so invaluable that it actually makes it possible for this 'trans-material' transmutation, that somewhat defies the Law of Natural Providence.