In default setting wordpress wont import xml file more than 2 MB but if you have access to your php.ini file you can just set the following variables:
upload_max_filesize = 20M
post_max_size = 20M
max_execution_time = 200
max_input_time = 200
in another way, using wpsplitter that you can downloaded from here to split xml file, just upload and thats working 
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export wordpress database more than 2 MB