Provided this isn't happening because you're feeding a lot of wet veggies and your rabbit is getting adequate fluids from that and that's the cause of the reduced drinking (unlikely with there being significantly reduced poop size), the likely causes are usually going to be a problem with the water or a health issue.
Though a sudden change in the rabbits environment or life, like a new pet in the house, predators lurking around, noisy environment, or other stressors, can also affect a rabbits eating and drinking. So you need to evaluate if there might be something causing your rabbit to be unusually stressed. Though typically if stress was the cause, eating would also be affected.
Sometimes rabbits can be picky about the taste of their water and any sudden changes to it. Has anything about your water changed, like change of home, new source of water, from a well? Even with no change of source it could happen, like with changes to how it tastes from the chemicals added in city water, or changes in well water and especially if any harmful bacteria growth has happened.
I would suggest maybe trying some bottled spring water. You could try a few different types, and if your rabbit still won't drink very well, it's possible there's another cause like a health issue. Pain from a dental problem is one possible common cause for this. Bladder issues like a UTI and bladder sludge, or kidney stones or disease, are some other possible causes.
https://wabbitwiki.com/wiki/Urine
https://wabbitwiki.com/wiki/Bladder_sludge_and_stones
https://rabbitwelfare.co.uk/urolithiasis-urinary-sludge-and-stones/
You'll also need to very closely monitor eating, as a rabbit not drinking well will also not be able to eat very much. And this can cause a slowdown of digestive motility, which you're seeing with the decreased poop size. Too much of a slowdown and you risk GI stasis setting in. Which if this happens, you won't have a choice but to immediately get your rabbit seen by a rabbit vet for treatment, sub q fluids, and meds. So you may need to help supplement your rabbits water intake temporarily. Syringing anything needs to be done carefully and slowly to reduce risks of aspirating. If in doubt, consult with a knowledgeable rabbit vet.
https://wabbitwiki.com/wiki/Gastrointestinal_stasis
https://wabbitwiki.com/wiki/Veterinary_emergencies
https://wabbitwiki.com/wiki/Choosing_a_rabbit_veterinarian