Electrical boxes are rated in cubic inches, and that number appears stamped inside plastic boxes or in the manufacturer's tables for metal ones. Everything placed inside consumes some of that volume according to a defined count, and exceeding the total is a real limit rather than a suggestion.

The counting rules are less complicated than they look. Each current-carrying conductor entering the box and terminating there counts as one. A conductor that passes through without a splice also counts as one. All equipment grounding conductors together count as one, no matter how many there are. All cable clamps inside the box together count as one. A device such as a switch or receptacle counts as two, based on the space its yoke occupies.

Each of those counts is then multiplied by a volume figure that depends on conductor size, larger conductors requiring more room, and the total has to fit within the box rating.

The reason for the limit is heat and mechanical damage. Conductors crammed into a box are difficult to fold without stressing the insulation, and pushing a device into an overfilled box can nick a conductor against a sharp edge or loosen a terminal that was tight a moment earlier. A packed box also traps heat that would otherwise dissipate.

This is the specific problem that appears whenever an old shallow box gets a modern device. A retrofit smart switch is physically deeper than the toggle it replaced, and adding a neutral pigtail on top of that can push a box that was already marginal past its rating.

The fix is usually to replace the box rather than to force the device. Old-work boxes with larger volumes, box extenders that add depth at the front, and deeper metal boxes where framing allows are all straightforward substitutions, and all of them are easier than diagnosing an intermittent connection later.

Ceiling boxes carry a second consideration alongside volume, which is what they are permitted to support. A box rated only for a light fixture is not rated for a fan, regardless of how much room is inside it, and the two requirements have to be satisfied separately.

Background reading: found this on Bjyou 4122