Increasing the number of teams at a World Cup sounds like a matter of adding matches. In practice it forces a redesign of the group stage, because the arithmetic rarely divides cleanly.
What a group stage has to achieve
Groups guarantee every participant several matches, which matters to nations that qualify rarely and to hosts selling tickets across many venues.
They also produce a ranked outcome that feeds a knockout bracket of a convenient size. The bracket must be a power of two for a clean elimination.
Those two requirements pull against each other, since the number of qualifiers must fit both the group structure and the bracket. Expansion disturbs the fit.
The problem with groups of three
Three-team groups reduce the number of matches each side plays and mean the final fixture involves only two of the three. The team already finished cannot respond.
That creates a situation where two sides know precisely what result eliminates the third. Collusion becomes theoretically possible even if it does not occur.
Simultaneous final fixtures, the standard safeguard in four-team groups, are impossible when one team is idle. The structural protection disappears.
Why groups of four are preferred
Four-team groups give every side three matches and allow the last round to be played at the same time. Each team controls its own fate on the final day.
They also produce a predictable number of qualifiers, which simplifies bracket construction. The format has been stable for decades because it solves several problems at once.
The cost is a larger total match count as the field grows. Tournament length and player workload both increase.
How additional knockout rounds change the tournament
A larger field usually requires an extra knockout round, which lengthens the path to the final. Winners play more matches in the same overall window.
That raises fatigue in the later stages, precisely where quality matters most to the audience. Recovery time between rounds becomes a design variable.
Extending the tournament collides with domestic seasons, which are agreed internationally. Every additional day has to be taken from somewhere else.
Ranking third-placed teams
Where some group runners-up advance alongside third-placed sides, those teams must be compared across different groups. The comparison is inherently imperfect.
Opponents differ in strength, so goal difference and points are not measured against equivalent tests. Tie-breaking rules become long and technical.
The bracket also cannot be fixed until the final group results are known, which complicates travel and ticketing. Administrative cost is part of the trade.
