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> IPv4 has a fixed header size (period)

Unless I'm mistaken, I would not say it's fixed due to the rarely used IP options field that varies depending on the IHL field. It has a variable size within a range. There might be a way to get "jumbo addresses" but it would have to be terribly hacked onto the design of the header as it is, which is one reason for IPv6's existence.

https://en.wikipedia.org/wiki/Internet_Protocol_version_4#He...

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My understanding of options is that many routers and/or firewalls consider it a dangerous and deprecated field and are most likely to drop packets that use them, especially when they are variably sized.

That also gets back to the point that almost all of IPv4 is too "known" as a design and every field accounted for in some router and/or firewall logic somewhere by the time IPv6 was designed.

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