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 w ithout 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.)