How can you build a terminal (T5) that isn't even big enough on day 1
1.Because of the physical restrictions of the site.
2. BA had to close the LGW hub due to unsustainable losses and so a swathe of long haul, (South America and part Africa) was moved back to LHR
BAA and BA had initially expected the BA operation to be smaller.
The planning process is the reason T5 is on so many floors, they had to build up.