1.5 lines of spacing will be larger with a larger font (compared to a smaller font). I am guessing that is not the issue in your sample.
A more likely explanation is that the document may contain blank paragraphs, added because someone pressed Enter twice. To find out, click the ¶ icon on the Home tab to display nonprinting marks. Para marks are represented by pilcrows, ¶.
If you can't get it sorted, consider sharing a representative sample document with the forum. Make sure that the sample does not contain any sensitive information that you do not want to share. You can upload your sample to OneDrive (or Dropbox) and post a download link in this thread.