I have written before that although we live in Canary Wharf in inner London, from whose high buildings you can just see the St Pancras rooftop, in the days of Ebbsfleet we found it notably more convenient, and quicker, to use that - just a few traffic lights between the two, park next to the station building, etc. However, it always seemed virtually deserted, the check-in, border and security staffs (three separate sets of course and no job sharing) always seemed so under-employed, and a complete opposite of the crush at St Pancras. I never saw the floorspace allocated to Eurostar in the two terminals, but would not be surprised if they were equal, or even Ebbsfleet had more.
Every single time we returned we found non-UK passengers trying to disembark onto the platform at Ebbsfleet, thinking they had arrived in London, and had to correct them and put them back on. I presume the staff had some standard procedure for dealing with this.