HS2 services will run to Manchester (and similar oop north places) from day one via the connection near Lichfield and the existing network, so they need to fit the existing infrastructure.
Trains built to a larger size would be stuck shuttling between London and Birmingham.
(The "HS2 will run only from London to Birmingham" thing is weirdly common in news articles, despite having never been true)
HS2 services will run to Manchester (and similar oop north places) from day one via the connection near Lichfield and the existing network, so they need to fit the existing infrastructure.
Trains built to a larger size would be stuck shuttling between London and Birmingham.
(The "HS2 will run only from London to Birmingham" thing is weirdly common in news articles, despite having never been true)