I just found the solution to solved my problem.
1. Login to wordpress admin page.
2. Goto settings –> Media and then change the midium size large number than default
3. Goto plugins and add new plugins name “Regenerate Thumbnails”
4. After finish activate plugins, goto Tools –> Regen thumbnails –> Click Regenerate All Thumbnails and wait a few (depend on how many photo on your site)
5. Go to your portfolio page……. If your portfolio page still not much better, try to redo step 2 – 4.
Good luck…… 🙂