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28.7% would be the theoretical limit for a massive consolidation tx. But consolidation is harmful to privacy, at least in the usual case of a single owner, as it signals common ownership of all the inputs.
28.7% would be the theoretical limit for a massive consolidation tx.
28.7% of the inputs. You cannot get 28.7% savings for the whole transaction. That's what I was trying to clarify.
By massive consolidation tx I mean a tx where the total weight of the tx is dominated by the weight of the inputs. E.g. hundreds of inputs and a single output. For such a transaction you can approach the 28.7% savings limit.
oh! like a 1 output tx. yes good point, sorry. (i was focusing on coinjoin as the likely reason for a huge transaction).
Further to that of course: coinjoin aside, just considering "batching" - that is by far the main concept in people's head when they think about CISA.
You're still very right to raise it though, for a very unusual class of consolidation tx, it is indeed possible to get that much higher (but still small!) saving.
To clarify in case a reader is confused, 28.7% of the input weight is a much lesser percentage of the total weight of the transaction, depending on the ratio of inputs to outputs (in size/weight).
Savings is basically 16.5 vB for every input except 1. Let's take a big coinjoin like 100 in, 100 out. You save 16.5 x 99 so 1633.5, but the original full size was 10.5 + 43 x 100 + 57.5 x 100 = 10060.5, so your percentage savings is 16.2%.
(That's already very close to the asymyptotic best, so I don't think you can get above like 16.5% however big the tx is, and for smaller transactions the savings can be a lot lower).