Well done on getting tickets, that's the greatest hurdle!
Standard pitches in East Campervan Fields are only for campervans/motorhomes 8 Metres and under. As you mentioned they had a large pitch in Bath and West I suspect they have a motorhome over 8 metres long? Or maybe they have purchased a smaller motorhome since their last visit?
Here is the information from the Festival Website - https://www.glastonburyfestivals.co.uk/info/#accommodation--campervans-and-caravans
Camping behind a campervan/ motorhome is allowed in East Campervan Fields but is very much subject to space. Shout out if you have any further questions. 😊