I finally got my Lt. Cmdr ship. A heavy cruiser only to see miranda teir 1 ships bigger than me. I think they should be made larger as they support 400 crew while a miranda class ship hold 200 people.
I think you encountered an optical illusion. I've looked at both the Mirandas and the Vesper/Excaliburs from every angle and next to lots of things, and the Vespers are demonstrably bigger.
It's sometimes hard to judge, because the saucer section of the vesper is close in size or even a touch smaller than the Miranda -- but bear in mind the Miranda's 'saucer' also contains Engineering, weapons and everything else that's in the neck and secondary hull on the Cruisers.
(Finally, the Tier 2 ships are not 'Heavy Cruisers.' They are 'Cruisers.' The Tier 3 ships are Heavy Cruisers. If it ain't got four nacelles, it ain't heavy. )