I’ve hunted around the internet for answers on this and it seems to be pretty subjective…so might as well ask here as well!
I’ve put 25/32mm inner tubes in 42mm tyres on the road/gravel bike and had one pop at the weekend and no sign of tyre damage or anything poking in.
Putting the tubes in and inflating them it did feel like they were too small – eg inflating tight against the rim while the rest of the tyre felt quite baggy, until inflated even further.

Internet search seems to suggest that smaller inners in larger tyres are fine – but can get trapped etc.
Anyone any experience of this – are there any ‘rules’ about tyre and inner tube size?

you’ll find you need to run highish pressures to force the tube to expand enough to actually press against the inside of the tyre and then the rubber of the tube is stretched quite thin so more likely to pop or tear.

Try inflating a tube by itself, you’ll find it stretches very easily, so I really doubt there will be any pressure difference once installed. I’ve seen 26” MTB tubes installed on 29” tyres with no issues whatsoever.

The only downside is the tube will be stretched, so it will be thinner and more puncture prone.

had one pop at the weekend and no sign of tyre damage

As the tyre rolls there is a scrubbing action that occurs at the tyre/tube interface which adds to rolling resistance and is one of the reasons for running tubeless.

The two surfaces scrub across one another when the wheel rolls and, depending on the materials, one will be stronger than the other. When the tyre wins the battle, the tube fails. When the tube wins, the rider enjoys a great setup that lasts a long time.

put 25/32mm inner tubes in 42mm tyres

or looking at it another way the tube is inflated 168% from its minimum

Yup, all that. It <can> make a difference with really extreme stretches, I’ve only ever experienced it with a fatbike tyre and a DH tube- at the desire pressure teh tube was definitely exerting a lot more pressure on some parts of the tyre than teh other, it looked and felt weird. You can’t recreate that with any sort of normal tyre but you never know, there might still be a less significant impact.

It’s the punctures thing that’ll actually make the difference though, it ends up exactly like putting a super light tube in.