[eluser]Lewis Cowles[/eluser]
Use models with functions for business logic for specific components that you have more than one of and may, or may not be database driven.
Use libraries or helpers for shared concerns between models that handle logic, but should mainly comprise of static classes.
Also try to cut out any unused data, I'm working on a bespoke ERP SaaS project at the moment for a very specific business and my CI, using this method is only about 10MB (Half the memory footprint of a vanilla wordpress install), although the raw application data footprint is several gigabytes
