Housing standards for pets and captive animals are usually written in terms of minimum dimensions. Behavioural evidence points consistently towards what is inside the space rather than how large it is.
Empty space is not used space
Animals given a larger bare enclosure typically use a small portion of it, concentrating near shelter, food and the boundaries they patrol.
Floor area only becomes usable when it contains structures that give a reason to occupy it, such as cover, height, substrate or foraging opportunity.
Adding complexity to a small enclosure therefore often produces a larger behavioural change than doubling the area of an empty one.
Control over the environment is the key variable
Being able to retreat, hide, choose temperature or avoid a companion reduces measured stress responses more reliably than most other modifications.
The presence of an option matters even when it is not used, because predictability and the ability to escape are themselves what lower arousal.
This is why a hide box in a small enclosure can outperform a much larger open space with nowhere to withdraw to.
Foraging occupies the time that would otherwise be empty
Wild animals spend a large share of their waking hours searching for and processing food, and a bowl removes that time entirely.
Scattering food, hiding it, or presenting it in a form that requires work restores part of the sequence and fills hours that are otherwise unstructured.
Repetitive stereotyped behaviours in captive animals are strongly associated with that unstructured time, and foraging enrichment is among the most consistently effective responses.
Enrichment has to change or it stops working
Animals habituate to novel objects within days, after which the object is present but no longer produces investigation or use.
Rotation, variation in placement and unpredictable timing preserve the effect, which makes enrichment an ongoing routine rather than a one-off purchase.
Not all enrichment is beneficial either, since items that cause frustration, injury risk or competition between animals can raise stress rather than reduce it.
Species differ in what the space needs to provide
Vertical space matters enormously to climbing species and very little to ground-dwelling ones, so identical dimensions serve them completely differently.
Social species need enough space and structure to maintain distance from each other, which means the requirement scales with group composition rather than with headcount alone.
Standards written as a single floor area figure inevitably miss this, which is why welfare assessment increasingly uses behavioural indicators rather than measurements.