A top Pentagon official has focused new attention on a French investigation of illegal exports to the Soviet Union by describing the case as rivaling the wrongdoing by Japan's Toshiba Corp. Stephen D. Bryen, deputy undersecretary of defense for trade security policy, told a group of Pentagon reporters Wednesday that U.S. authorities have been deeply involved in investigating a "serious" technology diversion case. Bryen, claiming he had to be "cagey" in releasing information at this point, refused to identify the company under investigation. He said only that American investigators were receiving "splendid cooperation" in the matter; that some individuals already had been charged and other arrests would follow, and that he hoped to make more information available in a few months. Other government sources said the investigation concerned a French company known as Forest Line that once went by the name of Ratier-Forest.