If I recall correctly write heads are smaller than read heads so when a rewrite need to occure in a single cluster, for one read to update the cluster you have 2 or 3 writes to update the overlapping blocs

Write heads are larger than read heads which is why SMR writes are problematic. This is why reads are not affected. The read head can read an overlapped track because it only needs a portion of that track. The write head is wider, which is why entire sections need to be read and rewritten elsewhere to implement a change in an SMR zone.

It doesn't matter if it's small files or big files, they're still just sectors. An entire SMR zone has to be read, written elsewhere, and then clear out the previously written SMR zone so it can be used again.