The size and blooming of Alpinia purpurata/purpurea can be influenced by many factors.

We've seen them here growing to 10+ feet tall. We have newly planted ones blooming right now at about 1 foot and 3 feet tall. Oftentimes, you'll see the babies, less than a foot tall, blooming while still attached to the mother plant.

Some plants grow quickly, others seem to take forever. Even the "same" plant in different parts of the yard grow differently.

I think it depends on all factors: sun, water, soil, fertilizer, length of growing season, etc.

Remember, Alpinias are a zone 10-11 plant. Perhaps in zone 7 they're not getting a long enough growing season.

-Ray