You're calling  $this->getLayout()->createBlock('sample_bigcom/adminhtml_form_edit_tab_form')->toHtml() But your block class is  Sample_Bigcom_Block_Adminhtml_Form_Edit_Form you are missing the tab in your class.

You're calling 

$this->getLayout()->createBlock('sample_bigcom/adminhtml_form_edit_tab_form')->toHtml() 

But your block class is 

Sample_Bigcom_Block_Adminhtml_Form_Edit_Form

You're missing the tab in your class and make sure that's where the file lives as well so Sample/Bigcom/Block/Adminhtml/Form/Edit/Tab/Form.php.