Seasonal weight fluctuations are not uncommon in pets (or people, for that matter). The long, warm days of spring and summer offer more opportunities for exercise than the inclement nights of winter.
As well, some degree of weight and muscle loss naturally occurs in all individuals as they age. An 83-year-old person can be expected to have less muscle mass than a 23-year-old; the same is true in cats and dogs.
However, and this is a big however, 16-year-old cats are at increased risk for a number of diseases and syndromes that may cause visible weight loss. Hyperthyroidism, kidney disease, tumors and a host of other glandular and metabolic problems are prevalent in older animals. Hyperthyroidism is especially known for causing weight loss and high levels of vocalization.
Your best move will be to have a vet examine your cat. He or she will be able to quantify the weight loss and run tests to check for the various syndromes I have mentioned. Hopefully the vet will determine that nothing is wrong. But it is better to be safe than sorry.