Svelte means slender. It's used to describe people, not things, and it implies a certain elegance. You might say that a middle-aged woman who had kept a svelte figure could still pass for a sixteen-year-old girl.
Svelte came to English from the French, back in the 19th century when the French had the last word on fashion, which probably accounts for its associations with elegance. It has nothing to do with the word sweltering, which means hot enough to make you sweat.