I am referencing both the Star Wars Rebels Insider information at the time, which stated it was "Significantly Larger", and "Not the Immobiliser Cruiser of Old."
The Armada Model is a good reference for it - as it is a spitting image of the Rebels Version - also has the centerline trench system that the Immobiliser does not have.
It also places its size as "Larger than a Victory" in the scaling system - but it still is smaller than an Imperial... But the Victory is still significatntly larger than the 418...
There are numerous small differences, moreso than just size.
I am mostly referencing what, y'know, the Official Sources themselves have said on it.
A Death Star by any other name will still a planet destroy.
So an Interdictor would, were she not an Immobiliser call'd, retain that dear ability which she owes without that title.
Interdictor, doff thy designation, and for that name which is no part of thee, pluck out from their safety at the speed of light all Force-forsaken Rebel scum.
(Bigger, newer: makes no difference. We know a descendant of an older concept when we see one.)