iguano

Both witness data and the rest of the transaction data count toward a single limit. That limit is 4 million weight units (WU) or, equivalently, 1 million virtual bytes (vB). Traditional transaction data counts as 4 WU (1 vB) per actual data byte. Witness data in segwit transactions counts as 1 WU (0.25 vB) per data byte.

Thus, a block can include up to 1 MB of non-witness data or up to (almost) 4 MB of witness data, but not both at once. If you have 3 MB of witness data (= 0.75 MvB), there's only 0.25 MB left for non-witness data.

So in order to reach the theoretical maximum of (almost) 4 MB in a block, you need the block to consist almost entirely of witness data, with only some minimal transaction data (some 100 to 200 B).

shafemtol

bitcoin

Well hard to explain, there is some guide already on internet that teach you how Segwit works .
But on a high level explanation I can say that the only part of the transaction who is outside the 1mb limit is the signature , so in order to get a 4mb block you need a signature of 3mb. 