In a more general context, yes, first jhana can be described as ekaggata. But in the gradual samadhi in the 4 jhanas formula, 2nd jhana is distinctly more powerful, and as one is first learning the jhanas, 2nd jhana can be exponentially stronger, felt in the piti-sukha, as the similes in AN 5.28 aptly describe.
On the other hand, some say the progression from 1st thru 2nd and 3rd can be seen as relatively subtle gradation, but the transition to the 4th is more drastic, and difficult to attain – similar to the marked change upon attaining the 1st jhana.
In purified jhana the mind is like water being purified from sand particles. Each vat/jhana purifies it some more:
To mix the metaphor going from hindrance free samadhi to the first jhana is like water suddenly freezing and turning into ice. The force of samadhi would be the binding force of the ice compared to water and this builds up to the fourth jhana.
The fourth jhana is like the ice made up of purified water. It has awareness and equanimity.