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.
As an example, see Rav Hirsch explaining the meaning of each plague and how it fits into the three groups of punishment.
The miracles of the Yam Suf, showed a completely different level of control in that it was not just an intensification of an aspect of the natural world, but a symbolic repudiation of all the laws of nature. Thus it was a manifestation of Hashem's hand, the complete manipulation of nature in a single occurance.
The Hagada compares the "finger of God" in the context of the ten makot to the "hand of God" mentioned at the Red Sea. A hand has 5 fingers, so 10 plagues x 5 fingers = 50 Red Sea "plagues".
But there had only been 3 makot so far when the "finger" phrase was used, and the phrase was referring to only 1 of those 3. So either 3x5=15 or 1x5=5.
– Daniel Apr 13 '17 at 21:54