Order-book depth is not pool liquidity

Someone comparing a centralised exchange with a decentralised one usually reaches for a single figure: this venue has more liquidity than that one. Both numbers are real, both are measured honestly, and putting them side by side still compares a signed contract with a stated intention.

Liquidity in an AMM pool is capital that has been transferred into a smart contract. While your trade executes, it is there. Nobody can withdraw it mid-transaction, no one decides whether to honour it, and the price it gives you follows from the invariant whether the market is calm or on fire. The provider can remove it afterwards — and the removal is itself observable — but not during, and not selectively for you.

Depth in an order book is a set of limit orders. Every one of them is an offer its owner may cancel, and cancelling is free, instant and unlimited. The size you see is what people are currently willing to do, not what they have committed to do.

Most resting depth on a large venue belongs to market makers, and a market maker's business is to earn the spread while avoiding being on the wrong side of an informed trade. When flow turns one-directional — the signature of news, or of forced selling — the rational response is to widen the spread or step back entirely.

So the book is deepest when nothing is happening and thinnest at the moment size arrives. This is not misconduct and mostly is not spoofing. It is a quote being withdrawn by someone who was never obliged to keep it there, doing what the arrangement always permitted.

A pool has no such reflex. It cannot see who you are, cannot tell an informed trade from an uninformed one, and cannot decline. That is a genuine cost to liquidity providers — they are picked off by better-informed flow, and impermanent loss is partly the accounting of that. It is also the reason the depth is still there in the minute it is most needed.

The comparison worth making is not which number is bigger but which kind of number it is. Ten million dollars of resting bids is a fair description of this instant and a poor description of the next one. Ten million dollars of pool reserves is a fair description of both, because withdrawing it is a transaction that itself takes time and can be seen.

This is also why cascades belong to the order-book side. A forced sale into a book that has just widened finds the gap the previous article described. The same forced sale against a pool moves along a curve — expensively, sometimes brutally, but continuously, and by an amount anyone can compute in advance from the reserves.

None of this makes pools safer. Their capital can be removed once the trade is over, their prices depend on arbitrageurs bothering to show up, and a pool with little in it is thin in a way no amount of contractual bindingness repairs. The claim here is narrower and worth holding onto: one number is a commitment, the other is an intention, and no headline that adds them together is telling you what it appears to.