I think there are two things in play here:
The confusion about whether 2m distances are required within bubbles (for older than EYFS, as they have specific space requirements), or only between them, and the maximum size of the bubbles.
At present, only some schools are doing the full 'maximum' of 15. Many are restricting the maximum size to the number who can be 2m apart, which takes bubble size down to 6 or 8 in many classrooms, meaning a standard class of 30 could need 4-5 rooms + 4-5 staff at minimum.
If the guidance was absolutely explicit that 2m within bubbles should not be used as a way to calculate numbers, then a standard class of 30 requires 2 classes and a minimum of 2 staff (though tbh as no other adults can come into the room, 2 staff per room is a much more workable ratio, even for Y6).
This still means that in a standard primary with 7 year groups, the return of 3 year groups occupies the same space as 6 year groups usually would - which with keyworker children in the space of the 7th year group easily means that the school is full.
Only if 2m spacing within bubbles is completely removed AND maximum bubble size = normal class size can more year groups return, as only then do any classrooms become vacant. Even that requires essentially a full school staff - remove a few who are shielding and it becomes impossible.
Equally, rationed / rota-ed use of toilets - often the only place hot water is available for handwashing - may pose a practical maximum of children in at any time, simply for 'throughput through the toilets / sinks without mixing bubbles' reasons.
Alternatively, everyone - including those now returned plus keyworkers - go onto a half time timetable, though all staff would have to be full time and be in charge of both halves of each class.