Sometimes we use Ant to compile the Flex Builder projects, but the compiled application/library couldn’t be used normally although it is normal under Flex Builder. Generally, the compile result of Ant is smaller than Flex Builder’s. Adobe support center has done the specification and supplied the solution. If you met this problem, following document for your reference.
ANT is an excellent tool for automating software processes. It is like MAKE but it uses the Java language. If you are using Flex SDK not the Flex Builder, it must need to repeat the same command to complete the compile and package work. Then ANT could help you to do so. Following is the article introduce how to use ANT to compile your SWC( an example showed ). You could have a try according the example.