Thank you @Rea_M 
"You're on the right track in checking your client's list information and file size. QBDT is designed for small to mid-sized businesses".  
Would you classify this as a small to mid-size business?
The link to the maximum number of List entries doesn't say anything about 2.2million targets in 12 months. Is this typical?
How have you helped other people resolve an issue there a database is at 1.4Gb after 12 months? There is no opportunity to keep at least 2 years of transactions.
What would you recommend as the correct version of QB for a business that has generated these figures after 12 months trading please?
I know I am asking many questions,  but as you are QuickBooks Employees, you are in the best position to give me the correct solution going forward.  Or in your opinion, is QuickBooks the solution that I need for this business?

Kind regards
Denis