Extraction happens at a surface
Water can only dissolve what it is in contact with, so the rate at which a tea extracts depends heavily on how much leaf surface is exposed per gram. Cutting a leaf in half creates new exposed faces. Cutting it into small fragments creates a great many. The relationship is steep: as particles get smaller, the surface area per gram rises sharply, which is why the difference between whole leaf and dust is not a matter of degree in the way it looks. Two teas of identical origin, identical processing and identical mass will extract at very different rates if one has been cut and the other has not.
What that means at the mug
Fine broken leaf reaches full strength in a fraction of the time whole leaf needs, and then keeps going straight into over-extraction while the clock still says there is time left. This is why bagged tea, which is generally made from cut leaf precisely because it must brew fast in a confined space, has a narrow window and turns harsh if left. It is also why a packet instruction written for one grade produces a bitter cup applied to another. Grade is not a quality statement; it is a statement about geometry, and geometry is what your timing has to match.
Fines, sediment and grip
Even a whole-leaf tea contains some dust, and more accumulates as leaf is handled, scooped and shipped. Those fines extract far ahead of everything else and contribute disproportionately to astringency and to cloudiness in the cup. This is the reason a first rinse is sometimes described as washing rather than waking the leaf, the reason a fine strainer improves a cup of broken-leaf tea noticeably, and the reason pressing or squeezing leaf makes the drink harsher — pressure forces out precisely the concentrated liquid held among the fines.
Compression works the other way
Compressed tea inverts the problem. A tight cake or brick has very little accessible surface until it has been separated and wetted, so it extracts slowly at first and then accelerates as it opens. This is why compressed tea benefits from a rinse, why it tolerates and often needs boiling water, and why it can produce so many infusions — the interior is still arriving at the party several steeps in. Prising leaf apart along its layers rather than chopping through it matters for exactly this reason: chopping manufactures fines and destroys the slow release the format provides.
Powder is a different regime altogether
Milled tea is not the extreme end of this scale, it is off it. At matcha’s particle size the material stays suspended rather than settling, and it is drunk rather than strained out, so nothing is left behind at all. The variables stop being about extraction and become about suspension and dispersion. That is why whisking technique, sieving and water temperature matter so much and steeping time barely exists as a concept — there is no separation step to time.
Adjusting for a grade you were not expecting
The practical rule is that finer leaf wants shorter time, and coarser or compressed leaf wants longer time and hotter water. If you open a packet and find much smaller particles than you expected, cut the steep before you brew rather than after. If the leaf is unusually large and intact — big white tea leaves, twisted whole-leaf blacks, rolled oolongs — expect it to need longer than the instruction, and expect the second infusion to be better than the first because the leaf has finally opened.