[jlrubin]

No, it is a space increase! Witness data is counted with a discount. Blocks can be larger than 1MB under SegWit.

[Kinnard]

Perhaps this is semantic?

[jlrubin]

It's not semantic, it is substantive. A maximally utilized SegWit block is larger than a maximally utilized current block.

I.e., if you were planning out how much disk space you needed years in advance, you would have to increase that figure non-negligibly if SegWit activates.