Broad interpretation of the Commerce Clause

[politicus]

How has it been possible to interpret the The Commerce Clause to justify so many and so varied exercises of federal power? I fully understand the political rationale behind this development (given how hard it is to change the Constitution), but it just seems to be obvious that it is objectively wrong to interpret it in such a broad manner. Even if you don't subscribe to original intent and consider the text a "living document", one should assume that there are some limits to how far you can stretch the Constitution.
A verdict like Wickard vs. Filburn seems to be completely unreasonable. So did the justices basically just rule based on their own political views?  

I have found this list (on a right wing site, but it seems factual - though maybe no 6 is an exaggeration). Once you pass no. 3 it becomes clear, that you leave a common sense interpretation of commerce and engage in "stretching".

In generally ascending order of breadth, various writers and Justices have defined “commerce” as:

1) The trafficking and trading of economic commodities
2) The trafficking and trading of economic commodities and the modes of their transportation
3)The trafficking and trading of any kind of commodity and the mode of its transportation
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4) The movement of any thing or any person and its mode of transportation
5) Economic activity that substantially or causally impacts on the trafficking, trading, or transportation of commodities
6) Any human activity or other phenomenon that has any ultimate impact on activities in more states than one.

[politicus]

That's a silly way to look at this issue.  Regulating commerce necessitates regulating both commerce itself and anything that substantially affects commerce.  So, the definition of commerce is basically meaningless in the grand scheme of things.

That's the crux of the problem, the Supreme Court has tried all these weird formulations, directly affects, substantially affects, they're all pretty useless definitions in practice.  So, instead we have a sort of functional definition defined by what falls within state regulatory power and what Congress decides to regulate.  That works fine and the people who argue otherwise are just conservatives who want the Constitution to constrain the government to a specific type of 19th century free market barbarism.

Its hardly silly, since spill-over of the Commerce Clause to other fields is the basis for most of the growth of federal power and its clear that this vastly exceeds anything that was intended with the clause. Without the Commerce Clause the relationship between the federal government and the states would have been quite different.

What interests me is how the stretching was done. Wickard vs. Filburn seems like a purely political verdict on the face of it, so I was uninterested in legal back story (precedents, rationale etc.) that led to this seemingly unreasonable verdict, not the well-known political context.