Framing the question properly
The comparison is usually made as a status claim, which makes it impossible to settle. Framed physically it is tractable: what does putting tea inside a sealed permeable envelope do to extraction, holding everything else constant? Three things change. The leaf is confined and cannot expand freely. The water has to move through a barrier to reach it and carry dissolved material back out. And the barrier itself may interact with the liquid. Everything else — the quality of the leaf, its grade, its freshness — is a property of what was put in the bag and could equally be put in a pot.
Confinement and headspace
Dry leaf expands to several times its volume when wetted. A flat two-chamber bag holds it in a thin layer that has almost nowhere to go, so the leaf packs against itself and water circulates around the outside of the mass rather than through it. This is the same failure as a cramped mesh ball, and it has the same signature: the tea is weak, and the response of steeping it longer produces something both weak and harsh. Tetrahedral bags exist to address exactly this and genuinely do, by giving the contents a volume to open into.
Particle size, which is the bigger factor
The leaf in a conventional bag is generally cut small, and that is not incidental — small particles are what allows a confined, unstirred, short brew to reach full strength at all. The consequence is that a bag brews fast and has a narrow window before it turns harsh. Most of what people notice when comparing a bag to loose leaf is this, rather than the bag: the same cut leaf loose in a mug behaves much the same way. That is why an empty fill-your-own bag holding whole leaf makes a perfectly good cup, and why the format cannot honestly be blamed for the grade it usually carries.
What the material contributes
Paper is the standard and it does absorb a small quantity of the liquid and some of the aromatic compounds in it. This is a real effect and a small one, and it is the same reason a cloth filter noticeably changes the mouthfeel of Hong Kong milk tea while a metal mesh does not. Woven synthetic mesh absorbs essentially nothing and passes more of the fine material through, which is a different trade rather than a strict improvement. None of these differences is on the scale of confinement or particle size.
The squeeze
Pressing a bag against the side of the mug is the most common attempt to improve a weak cup and it makes things worse in a specific way. What is forced out is the liquid held within the leaf mass, which is the most concentrated and most late-fraction portion available, along with a quantity of fine particles. The cup gets darker and grippier without gaining any of the qualities that were missing. If the cup is weak, the answer is another bag or more leaf, not more pressure on the one you have.
What the format is genuinely good at
Judged as a delivery system rather than as a compromised pot, the bag does its job well. It portions accurately, it needs no equipment, it produces a consistent result from a consistent product, it travels, and it leaves nothing to clean. Those are real properties that loose leaf does not have, and the format’s global dominance is not a failure of taste — it is a reasonable response to what most people need from most cups of tea. The honest criticism is narrower than the usual one: flat bags cramp the leaf, and the leaf inside them is usually cut fine.
The conclusion the physics supports
A bag imposes a real constraint, and that constraint is largely relieved by a roomier bag. The larger difference in most comparisons is the grade of leaf, which is a commercial choice rather than a property of the envelope. Someone who wants better tea and likes the convenience should look for whole leaf in a roomy sachet, or fill their own — and can stop treating the format itself as the problem, because on the evidence it mostly is not.