Why 3 kg of cat handles one chestnut easily
Your cat's small stature doesn't amplify risk here because chestnuts lack the dense fats or toxins that make other nuts dangerous in tiny amounts. Think of it as her eating a single bite of potato—it's just starch and fiber passing through. Even for a 3 kg cat, the piece you described is pharmacologically irrelevant; her liver won't need to process anything unusual.
Cats often swallow small morsels whole, which is actually safer with soft cooked chestnuts than with harder foods. There's no meaningful choking hazard at this size, and the lack of sharp edges means no internal scratching. The only way weight would matter is if she'd eaten a dozen—which could cause temporary bulkiness in the gut—but one is trivial.
The stealth advantage of peeled chestnuts
Because the chestnut was peeled, you've avoided the two real dangers nuts pose to cats: shell fragments causing obstructions and mold from improper storage (which can produce tremorgenic toxins). Cooked flesh alone is essentially a bland vegetable matter to a cat's system. It's less stimulating to their digestion than even commercial cat treats, which often contain stronger flavorings.
That said, cats lack the enzymes to derive much nutrition from plant matter, so this chestnut will transit her system mostly unchanged. You might notice the piece in her stool—don't mistake this for undigested food being problematic. It's normal for cats to pass fibrous plant material visibly since they can't break it down efficiently.
When small size does matter with scavenging
While this incident is harmless, a 3 kg cat's curiosity can get her into trouble with other human foods that are dose-sensitive—onion, chocolate, or xylitol would be dangerous at much smaller amounts than for a larger animal. Use this as a reminder to cat-proof snack areas, not because chestnuts are risky but because her petite frame makes her more vulnerable to truly toxic substances.
Monitor her interest in human food after this. Some small cats develop a taste for starchy foods after such encounters, leading to more persistent begging. If she starts hovering when you eat, redirect her with high-protein cat treats to reinforce better habits before they form.
The 24-hour window for tiny cats
With such a small amount ingested, any reaction would be immediate (choking) or appear within 12 hours (mild digestive upset). Beyond that window, you can consider the matter closed. For peace of mind, check her litter box once—normal stool means her system didn't even blink at the chestnut.
Contrast this with a 3 kg cat eating fatty nuts like macadamias, where even one nut could cause neurological symptoms. Chestnuts are in a completely different safety category. Her size makes her adorable, but it doesn't make this snack any more consequential than it would be for a 6 kg cat.
The full verdict, portion guidance and serving notes are collected on the Chestnuts food facts page.