W2 return, standard deduction.TurboTax Maryland State refund as $151. Maryland iFile website refund $341. Same W2...

Double check your entries on both forms to be sure there is not a typo on one of the forms.  Pay special attention to the decimal points.  

Also, can you see the form 502 for Maryland?  If so, you can compare the one from the state site with the one from TurboTax to see where the difference is coming from. A line by line comparison is the best way to figure out where the difference is coming from. 

On the menu bar on the left that shows. 

1. Select Tax Tools 
2. On the drop-down select Tools 
3. On the pop-up menu 
4. Select View Tax Summary 
5. On the left sidebar, select Preview my 1040 

If you are using the desktop version, you can simply switch to forms mode and then click on form 502 under Maryland on the left hand side.

Yes double check W2 box 17 for state withholding.  That usually gets entered wrong.