Grasso vs. paffuto vs. obeso
While grasso, paffuto, and obeso all describe someone with excess body fat, they are used in entirely different contexts. The choice between them depends on whether you want to state a general fact, express affectionate endearment, or make a clinical medical observation.
Grasso
The word grasso is the most common and standard Italian translation for the English word fat. It is a general, everyday term used to describe people, animals, or even objects that are overweight. Depending on the context and tone of voice, calling someone grasso can range from a purely neutral observation to being somewhat rude.
Da bambino ero un po' grasso, ma poi sono cresciuto e dimagrito.
(As a child I was a bit fat, but then I grew up and lost weight.)
Il mio gatto è diventato troppo grasso perché mangia in continuazione.
(My cat has become too fat because he eats constantly.)
Quell'uomo grasso seduto sulla panchina sta leggendo un giornale.
(That fat man sitting on the bench is reading a newspaper.)
Non mi sento a mio agio con questi pantaloni, mi fanno sembrare grassa.
(I do not feel comfortable in these pants, they make me look fat.)
Il medico le ha detto che è troppo grassa e deve fare attività fisica.
(The doctor told her she is too fat and must do physical activity.)
Paffuto
The word paffuto translates closely to plump or chubby in English. Unlike grasso, paffuto carries a very positive, cute, and affectionate connotation. It is almost exclusively used to describe babies, young children, or small animals that have soft, round, and healthy-looking features, especially their cheeks.
Il neonato ha delle guance rosse e paffute che tutti vogliono baciare.
(The baby has red and chubby cheeks that everyone wants to kiss.)
Il suo nipotino è un bambino paffuto e molto allegro.
(Her little nephew is a plump and very cheerful child.)
Abbiamo appena adottato un cucciolo paffuto e morbidissimo.
(We just adopted a chubby and very soft puppy.)
Le dita paffute della bambina stringevano forte il suo giocattolo preferito.
(The little girl's plump fingers tightly held her favorite toy.)
Quel gattino paffuto non fa altro che dormire sul divano.
(That chubby kitten does nothing but sleep on the sofa.)
Obeso
The word obeso is the direct equivalent of the English word obese. It is primarily a formal, clinical, and medical term used to describe an unhealthy and severe level of excess body fat. Because of its strong medical nature, calling a person obeso outside of a healthcare context is considered harsh.
Il paziente è clinicamente obeso e rischia gravi problemi cardiaci.
(The patient is clinically obese and risks severe heart problems.)
Una grande percentuale della popolazione mondiale è oggi considerata obesa.
(A large percentage of the world population is today considered obese.)
Il suo cane è diventato obeso a causa della totale mancanza di esercizio fisico.
(His dog has become obese due to the total lack of physical exercise.)
Esistono nuovi programmi di supporto per i bambini obesi nelle scuole pubbliche.
(There are new support programs for obese children in public schools.)
Il nutrizionista ha spiegato i rischi per la salute associati a un corpo obeso.
(The nutritionist explained the health risks associated with an obese body.)
Summary
To summarize, use grasso as your everyday descriptive word for fat. Choose paffuto when you want to affectionately describe the cute, plump features of a baby or small animal. Reserve obeso for clinical, medical discussions regarding severe overweight conditions.