Why Soundcloud and instagram company size is so different?

Soeaking of consumer apps - Soundcoud has roughly 900 employee, Instagram had 18 employees at the time of acquisition. Why do companies hire employee for on such a scale, when other companies get similar things done for lesser employees. 900 vs 18. Do consumer apps need to have hundreds of employees to be successful? how come plenty of fish managed to get acquired for 450million with only one full time founder/employee?