Why is my pipe trunking so big?

Due to the market trend of offering upgraded material with 1/2″ thick insulation and 16mm Drain pipe with 1/4″ insulation. The trunking size for system 4 are usually 2 sets of 4″ by 3″ trunking. (Refer to picture on the right). So if contractors are offering you material with 3/8″ thick insulation and 13mm Drain pipe with 1/4″ insulation, you will notice that it is 2 inch shorter (Refer to picture on the left). Anything smaller than this means contractor are squeezing the material into the trunking which will cause condensation issue. Insulation should be lightly packed and not squeeze to ensure best performance of the insulation.