Typo3 by default limits image width to 600 px. You can easily change this setting using TypoScript. These TypoScript can be usually located on the root Template record. If not you can try using the object browser to find the exact location.

You may have to change this in setup and in constants as shown below:

In CONSANTS:
styles.content.imgtext.maxW = 700

In SETUP:
tt_content.image.20.maxW = 930