''You've gained weight'' is a traditional compliment in Fiji, anthropologists say.

In accordance with traditional culture in the South Pacific nation, dinner guests are expected to eat as much as possible. A robust, nicely rounded body is the norm for men and women. ''Skinny legs'' is a major insult. And ''going thin,'' the Fijian term for losing a noticeable amount of weight, is considered a worrisome condition.

But all that may be changing, now that Heather Locklear has arrived.

Just a few years after the introduction of television to a province of Fiji's main island, Viti Levu, eating disorders -- once virtually unheard of there -- are on the rise among girls, according to a study presented yesterday at the American Psychiatric Association meetings in Washington. Young girls dream of looking not like their mothers and aunts, but like the slender stars of ''Melrose Place'' and ''Beverly Hills 90210.''

''I'm very heavy,'' one Fijian adolescent lamented during an interview with researchers led by Dr. Anne E. Becker, director of research at the Harvard Eating Disorders Center of Harvard Medical School, who investigated shifts in body image and eating practices in Fiji over a three-year period.

The Fijian girl said her friends also tell her that she is too fat, ''and sometimes I'm depressed because I always want to lose weight.''