Smaller Sigma in Microsoft Office Word

Have you ever been so frustrated to Microsoft Word's formula because it doesn't give you the one that you want? I had once, a small problem that really annoyed me when I wrote my thesis; sometimes I get a smaller summation (sigma) symbol , but sometimes it gives me a big one. It makes my thesis become so messy because the symbols are not uniformed.

Here's the problem and the solution as depicted in the following picture:

Hopefully, this will help someone or save someone's time in the future!