Every venue shows a headline number: a spread, a fee, a markup. It is true for some order. The question is whether it is true for yours. Size changes cost, and it changes it in different directions on different kinds of venue.
On an order book, cost rises with size
The spread you see is the gap between the best bid and the best ask. Only a limited amount is available at those two prices. An order larger than that takes the next price, and the next. The average fill gets worse as the order grows. This is price impact, and the displayed spread does not include it.
For a small order in a large coin the effect is nil. For a large order in a small coin it can be the biggest cost of the trade. A deep book is not necessarily a liquid one covers why the displayed size is itself not a promise.
On a tiered fee schedule, cost can fall with size
Exchanges that charge a commission usually tier it by volume. Trade more in a month and the rate drops. The headline rate on the fee page is often the first tier, the one a new or small account pays. A large account pays less. So the published rate overstates cost for some traders and is exact for others. Reading a fee schedule properly goes through the tiers.
On a markup venue, read the disclosure
A broker that prices with a markup quotes you a price for your order and fills it there. Whether the markup is the same share at every size is a matter for that venue’s own disclosure. Where it is a flat share, the rate holds and only the money amount grows. The point is to check, not to assume it behaves like a book or like a tiered schedule.
On a pool, size is everything
An on-chain pool prices each trade along a curve. The larger the trade against the pool, the further along the curve it goes, and the worse the price. The price shown before the trade is for a very small amount. Position size against pool depth puts numbers on it.
A fixed fee turns it around
A network fee is charged per transaction, whatever the amount. On a small trade it is a large share. On a large trade it is small. So there are costs that fall with size and costs that rise with size, and the total has a low point somewhere between. Gas fees are a trading cost covers that trade-off.
The habit
Before using any quoted cost, ask what order it describes. Then work out the figure for the order you intend. The NoVo Crypto Market Map ranks what a round trip costs on each coin from Robinhood’s own disclosed markup, and maps on-chain tokens by pooled depth. The first is the stated rate. The second tells you how far a given size will move a pool.