When you utilize an "External Pricing Engine" option to pull pricing from another system that is not Pressero, the weight of that product is not pulled from the external source. This is often because the external sources that contain the pricing do not specify the weight either. Instead, this must be defined within Pressero when assigning the external pricing engine to the product.
When you edit the product and go to the pricing engine, you would define the pricing engine that pulls from an external source. Once this is selected, Pressero will present you with new options including a "per piece weight". This is where you define the weight per piece - also shown in the screenshot below.