Q:

Unable to import multiple .cmp files

I have run the command
 Export-SPWeb -Identity  $url -ItemUrl $largeList -CompressionSize 15 -Path "c:\itemFolder\backup.cmp"

This results in 3 files.
 c:\itemFolder\backup.cmp
 c:\itemFolder\backup1.cmp 
 c:\itemFolder\backup2.cmp

Now when I try to Import these files the second two fail. Error message: "Import-SPWeb : Failed to read package"
Log file has another error message : 
 The file operation of c:\itemFolder\backup1.cmp was preformed out of order. The file was operated on as order 1 but was marked as order 0. Please ensure these files are operated on sequentially starting from 0

I do import-spweb once per file, in order. Why is happening? 

A:

When you export a site / web, it created one or multiple CMP files depend upon the Size of Site and value of FileMaxSize 

By default, the .cmp files are limited to 24 MB in size, although you
  can change this value using the FileMaxSize property. If set to zero,
  the value resets to the default. When site data exceeds the specified
  limit, site data is separated in to two or more migration files.
  However, if a single site data file exceeds the maximum file size, the
  operation does not split the source file, but rather it resizes the
  .cmp file to accommodate the oversize file. You can have any number of
> .cmp files.

When there are multiple .cmp files, the file names are appended numerically. For example, where the baseFileName parameter is "MyWeb", and where the export operation produces multiple .cmp files, the migration packages are named "MyWeb1.cmp", "MyWeb2.cmp", and so forth.
IN NutSHell, you dont need to import each file individually rather you just need to run the import. In your case your Import command should be like this...
Import-SPWeb http://site.com -Path "c:\itemFolder\backup.cmp" -UpdateVersions Overwrite

