The new trees were never "small", but were certainly smaller than the old ones, and with less-dense branches. Even now, it's fairly easy to see through the trees to the fireworks beyond for most of the year, even during full foliage seasons; the old olive trees had thick branches and year-round leaves. The new trees were also positioned to allow wider sightlines through the area
Currently, the trees are only about 5' taller than the ones in the middle of MK's hub, but due to the scale of DL, they appear to fill the space better. MK's hub is so wide open, with the nearest meaningful structures on the other side of the moat, that any mid-sized trees look tiny in comparison. The new tree in photos near Tomorrowland Terrace appears to be taller than the DL trees, but not as wide
March 2003
March 2004 (photo zoomed in more, so the difference isn't as noticeable)
And I stand corrected, the trees were actually replaced in March 2004, but it was still done in conjunction with the 50th fireworks. At that time they were testing all sorts of various effects (including a disco ball atop the Matterhorn that could be seen outside the park, Disney's very first projection mapping tests on the castle that were photographed by an overnight CM, and rumors of a 100' tall Walt hologram in the hub), so they were definitely planning for the show, even if it was a long way from reality