Why does antenna size change read distance?
Coupling depends on how much of the reader field passes through the tag coil, and a larger coil intercepts more of it. That is why a card sized tag reads further than a small sticker with the same chip inside. The gain is real, and it flattens once the tag is larger than the reader coil.
Intercepting the field
The reader produces a magnetic field that spreads out from its coil. The tag receives energy in proportion to how much of that field passes through its own coil, which depends on the area the coil encloses and on how many turns it has.
A coil enclosing four times the area, all else equal, intercepts roughly four times the field at the same position. That is why the physical dimensions of a tag matter far more than anything printed on its datasheet.
Why the gain is not proportional
Four times the intercepted energy does not mean four times the range, and the difference disappoints people who expect it to.
The field weakens extremely quickly as you move away from the reader coil. Gaining four times the energy therefore buys back only a modest increase in distance, because you are climbing a very steep slope. In practice, doubling the linear size of a tag produces a noticeable improvement, not a dramatic one.
Noticeable is often exactly what is needed. Moving a tag from unreliable to reliable is usually a matter of a small increase, not a large one.
Where the gain stops
Only the part of the field that passes through both coils contributes. Once the tag coil is substantially larger than the reader coil, the extra area is enclosing field that never went through the reader in the first place, and it adds nothing.
Since a phone antenna is small, this ceiling arrives sooner than people expect. A tag much larger than a credit card gains little over a card when the reader is a phone, though it may gain a great deal with a large fixed reader.
The practical consequence: size the tag against the reader you will actually use, not against the largest tag available.
What the chip contributes
Almost nothing, at this level. Two tags with the same antenna and different chips read at very similar distances, because the chip determines memory, features and price rather than coupling.
This is worth stating plainly because tag listings emphasise the chip and rarely give the antenna dimensions. When range matters, the number you need is the one the listing omits, and asking for it is a good test of whether the supplier understands their product.
Frequently asked questions
Will a bigger tag fix my read problem?
It is the cheapest thing to try and it works often. It will not help if the cause is metal behind the tag, since a larger antenna over metal is simply a larger antenna being detuned.
Does the number of turns matter as well as the area?
Yes, both contribute to how much field the coil intercepts. Manufacturers balance the two against the chip's requirements, which is why you cannot compare tags by dimensions alone across very different designs.
Why do stickers come in so many sizes if bigger is better?
Because bigger is not always possible. The tag has to fit the object, survive the application process and cost what the deployment can afford, and range is only one of four constraints.
Is a round tag better than a square one?
The enclosed area matters more than the shape. A round tag of a given diameter encloses less area than a square of the same width, which slightly favours the square where the object allows it.