Per the answer given here: אצבע אלוקים היא - *how* many?:
The magicians were acknowledging that the plages were a manifestation of Hashem's Power at the level of אצבע אלוקים היא. That is, the plagues as a general concept were a finger of Hashem, a small manifestation of His power. At the Yam Suf, the expression of the Power was that of a hand. It was a mightier manifestation of Hashem's control of nature and ability to overcome the laws of nature. Each of the plagues took an aspect of the natural world and showed that Hashem was able to manipulate it. etc.
I was wondering how the chartumim were able to measure Hashem's power-levels, so to speak, enough to understand that Hashem, in the first three plagues, was only revealing a small bit of His full might? It seems to me that for all they knew, the third plague could've been His ultimate power, further than that - and He'd be beat, chalilah.