Small teams routinely commit to more work than they can complete, then treat the shortfall as a productivity problem. The arithmetic used to make the commitment is usually the real cause.
Available hours are not usable hours
A full-time person does not supply a full week of project work. Meetings, administration, recruitment, support requests and interruptions consume a meaningful share before anything planned begins.
The usable proportion varies by role, but it is always well below the contracted total. Planning against the contracted figure guarantees an overcommitment from the first day.
Teams that measure their actual usable share, even roughly, produce far better forecasts. The number is uncomfortable, which is why it often goes unmeasured.
Specialisation removes flexibility
In a large team, capacity is somewhat fungible because several people can do each task. In a small one, individual skills are rarely interchangeable.
Total available hours can therefore look sufficient while the specific person needed is fully committed. The team is idle and blocked simultaneously, which averages hide completely.
This makes the constraint a named individual rather than the team. Planning has to be done against that person's calendar, not against the aggregate.
Unplanned work is predictable in volume
Support, escalations and urgent requests are unpredictable individually but reasonably stable in total. A team that has always absorbed a certain volume will absorb it again.
Plans that allocate every hour to planned work implicitly assume that volume will be zero. It never is, so planned work slips by roughly the amount that was ignored.
Reserving explicit capacity for it produces plans that survive contact with the month. The reserve is not slack; it is the part of the work nobody scheduled.
Absence is arithmetic, not misfortune
Holiday entitlement, public holidays, sickness and training remove a substantial share of the year. In a team of four, one person on leave removes a quarter of the capacity.
Larger teams smooth this because absences overlap and average out. Small teams experience each absence as a step change in what the team can deliver.
Because entitlement and statutory leave rules vary by jurisdiction, the deduction differs between countries even for identically sized teams. It has to be calculated locally rather than assumed.
Utilisation targets create queues
Planning a team to near-total utilisation removes its ability to respond. Any variation, and there is always variation, has nowhere to go except into a growing backlog.
Queueing behaviour is sharply non-linear near full load. Waiting times rise slowly as utilisation climbs, then rise very steeply once the system is close to saturated.
Which is why teams that look efficient on paper often feel slow to everyone dealing with them. The efficiency was purchased with responsiveness that nobody costed.