The point of local networks of a minimum size of 64 bit isn't only to have MAC-based addresses (48 bit would have been enough for that, fwiw), but in general to support non-coordinated/probabilistic self-assignment schemes with negligible collision probability.
Picking a random local address (which is very important for privacy, as you've mentioned) is much easier if you don't have to do an elaborate dance of listen, announce, listen for collisions etc. first (practically that still happens, but collisions are the absolute exception).
> So is the bigger address range better?
Yes, because consider the alternative of re-doing all of this again in a future in which IP usage for some reason jumps by a few orders of magnitude again.
Due to hardware getting better over time, the per-packet cost of a few extra bits is going down all the time, while the cost of rolling out a future IPv7 increases with every new deployed host.
The best thing about SLAAC is that it forces your ISP to give you at least 64 bits. Otherwise you know Comcast would only give out a /128 and charge you for more, so you'd use NAT at home just like IPv4.
Unfortunately SLAAC doesn't force upstream to provide a /64 universally.
Some ISPs are reportedly giving out a /128, and SLAAC works adequately with a router performing IPv6 NAT, so those ISPs don't see a problem.
Mobile phone as WiFi access point is another common way people access the net nowadays. I've occasionally seen permanent installations, with a phone taped to a window. I've never seen a mobile phone AP offer IPv6 to clients, but if they do they have to use SLAAC-compatible IPv6 NAT in that situation.
Thats why android supports dhcpv6-pd for a /64, but not assigning a /128 from dhcpv6
Well, my phone as access point grants an IPv6 public IP without NAT. There's a stateful firewall somewhere in the chain though.