Why did the destruction of the USS Kelvin cause the Enterprise NCC-1701 to be built at about twice the length in the new timeline decades later?

According to the Producers, the Kelvin took some scans of the Narada, that were then reverse-engineered by Starfleet, and incorporated into new ship design, and/or bidirectional changes in the timeline, stemming from the Narada Incursion, caused everything to be different.

Besides that, following the arrival of a new, highly-advanced alien starship, that was able to instantly blow away the Kelvin, Starfleet might have overbuilt their replacements afterwards, to try and mitigate the risks of that happening. The 23rd century would have been during the Klingon Cold War, so it's not impossible that Starfleet believed it to be a most-secret Klingon vessel that they had to prepare to defend themselves against.

Assuming Federation technology wouldn't have advanced all that much, the logical thing for them to do might have been to build the ships bigger, stuffing them with larger power plants/more systems under the hood. It would be the only surefire way of preparing themselves against either the Narada itself, or another power armed with ships like the Narada.