Other heating system changed after the heating load calculation was created

We are building a KfW40 house, the design of the heating system was done according to "we have always done it this way." However, I cannot equip such an insulated house with a heating system that, for example, is used in a KfW70 or "normally" insulated house. And we know this because everything was listed in detail in the contract, specifying what was installed differently and also more cheaply.

Unpack some details. What is guaranteed in the contract and what have you received now.

For example
Your Bosch air/water heat pump package
consisting of:
• BOSCH Monoblock outdoor unit AW 7 OR-S, series CS5800/6800i AW, 800x1100x550
• BOSCH air/water heat pump Compress, CS6800AW 12 wall-mounted indoor unit

etc.
It has now become smaller, which is cheaper

What does the developer say about the situation? After all, they are the contracting party.

He wants to shift that onto the heating engineer…

He wants to pass that on to the heating engineer…

Well, there is probably no contract between the heating engineer and you. Therefore, it is goodwill if the heating engineer pays you something.
Whether you have a claim against the developer depends on your contract. Since you wanted the simplified heating system, it is not necessarily the case that the reduced costs fall on you. Typically, that is then additional profit for the developer.