MaxItems lookup setting that controls the number of items in Lookup, Multilist and Valuelookup fields.

You have probably run into that your lookup field does not show over 100 items and the magic is in web.config and you can change to a higher value as desired!

  <!--  Query.MaxItems
            Specifies the max number of items in a query result set.
            If the number is 0, all items are returned. This may affect system performance, if a
            large query result is returned.
            This also controls the number of items in Lookup, Multilist and Valuelookup fields.
            Default value: 100
      -->
            <setting name="Query.MaxItems" value="100"/>