“The Or HaChaim explains:
[The Sages] say in the Zohar that by mentioning the abundance of good that someone else enjoys, one inflicts an evil eye upon that person and empowers the accusing angel to harm him; [However] the forces of evil have no authority over anything upon which one invokes the Divine Name.
Accordingly, we may explain that since Moshe had said in the previous verse, ‘I cannot carry you alone,’ he was effectively underscoring the abundance of [the Jewish people] by indicating that they were too numerous for him to judge on his own; and as a result, there existed the concern stated above, that they might be subjected to an evil eye.
Therefore, he immediately invoked the Name of [Hashem], blessed be He, upon them, and he said, ‘Hashem, your God, has increased you,‘ thereby ensuring that there would be no room for the evildoer (i.e., the accusing angel) to harm them, as stated there in the Zohar.
Accordingly, Moshe was saying is that their abundance was due to the fact that Hashem had increased them, not that an unusual increase had taken place.”