The 100k vbytes refers to the total size of the transaction including witness data, headers, the inputs and outputs, etc. For inscriptions spammers use the witness data aspect of the transaction.
From my understanding, each input has its own witness field, which can carry only a limited amount of data. To include larger files, the data must be split across multiple inputs and/or multiple transactions. The total transaction size cannot exceed100k vbytes.
How much bigger percent wise can one input in the mempool be after this? Is this a concern?
0%
max block size is 4mb since 2017.
There is no size increase for the total transaction (still 100k vbytes), and so there is no size increase to the inputs. The change is changing how much data in the op_return part of the transaction nodes will relay. The change means your op_return data can take up the entire 100k vbytes and core 30 nodes will still relay it by default. Prior to core 30 nodes would only relay transactions with a max of 80 bytes op_return data by default.
The change basically means the bitcoin p2p network supports transactions that are completely arbitrary data or spam.
The concern comes from whether you think nodes relaying 100kb files, uploaded anonymously for free could be an attack vector for bad actors.
