sam dilly
I fully sympathise with you regarding that problem.
The Immigration arrivals area is totally inadequate to accomodate the numbers of passengers waiting to get to the desks; even one A319 can result in people standing out in the cold. The good news is that a new arrivals hall should open in May, this being the first part of the terminal extension to be built as the problem is well recognised by the SEN management.
As to the reason for the current situation, I think it's fair to say that no one anticipated that SEN would be home to three A319s within a couple of months of the terminal opening. When the terminal design was approved the assumption was that 50 to 80 seaters would mainly comprise SEN's traffic for the first year or so until the terminal could be extended to cope with larger/greater numbers of aircraft. It's become a victim of its own success really.