Both these adjectives describe things that are much smaller in size or scale than is ordinary or normal. Miniature suggests something physically reproduced on a reduced, often tiny, scale: miniature dollhouse furniture; a miniature screwdriver. Diminutive suggests something or someone unusually small (the diminutive size of the phone; her diminutive stature). Although miniature is sometimes used to describe the size of adult people, diminutive is much less likely to offend. In fact, it’s a common word for describing both things and people that are formidable despite being small (diminutive wide receiver; the diminutive pop star).

Formal and ceremonious, when used to describe people rather than events, refer to conduct that observes the manners and conventions of behavior expected in certain settings or at serious or important events. Someone whose manner is formal is polite and dignified. When the situation doesn’t call for this, formal can connote stiffness or detachment: Why was she so formal with us? Ceremonious suggests someone who takes great care to observe the manners appropriate to solemn or important occasions: ceremonious manner; a ceremonious bow. As with formal, it can carry a negative connotation of excessive or exaggerated formality when the occasion doesn’t call for it. In novels from the past, many a farewell was taken with a slightly overdone ceremonious bow.

Plain and spartan both refer to the quality of something that is simple or spare and without any enhancements that provide pleasure, comfort, or ease. Plain suggests something basic, simple, and consisting only of what’s necessary, without pretention or embellishment (a plain room; a plain diet; plain packaging). Spartan suggests a stark simplicity of lifestyle, frugal and minimal to the point of harshness (a spartan diet; spartan conditions). The militarist society of Sparta in ancient Greece, whence we get this adjective, was renowned for its rigorous discipline in every aspect of life. Spartan is often used to describe rooms and furnishings (a spartan room; spartan accommodations).