Possible Clues:

• Dad is 'ample', greed possibly producing this?
• Mediaeval banquet might offer corporate growth
• Fifties phenomenon for many
• With inside condition, feast leads to flab
• Reason to diet and exercise
• Result of too many a medieval banquet?
• Result of too many a mediaeval banquet?
• An unwanted increase: 500 to 1500, say?
• In earlier times, power to study brings personal growth
• Extra fat butter? It's beneath e.g. lid made fancy