What designated peter frampton's frampton comes alive! a "big album"?

The answer is the fact that it was a platinum album which sold millions of copies.

It was April of 1950 when Peter Kenneth Frampton was born who later became a famous musician and singer. The man was previously linked with some other musical groups like Humble Pie and The Herd etc. Frampton Comes Alive was his breakthrough album which sold more than eight million copies in the US.