Summary:
The hitboxes of sniffers are not adjusted when they lay down.
When camels lay down, their hitboxes are adjusted accordingly. I'd expect similar behavior with sniffers; their hitboxes should be smaller when they lay down. This is also a parity issue -> MC-261417 was fixed in snapshot 23w18a. In java, sniffers' hitbox is now adjusted when they lay down (the hitbox becomes just under 1.5 blocks).

Steps to reproduce:

1. Build a small fence for sniffers.
2. Summon a sniffer.

3. /event entity @e[type=sniffer] on_scenting_success 

4. Try placing the top slab above the sniffer.

Observed results:
The hitboxes of sniffers are not adjusted when they lay down.

Expected results:
The hitboxes of sniffers would be adjusted when they lay down.

Screenshots:
Bedrock

Java