I think you miss Trice's point. There was more to the campaign than the siege. There were diversions and landings and battles fought. And opposing numbers at point of contact were surely not always overwhelmingly to the federals advantage.
Not always overwhelmingly, maybe, but in aggregate the force opposed to Pemberton is simply staggering - it's actually greater odds against him than Lee faced.
And the siege is what actually concluded the campaign, so if anything's to take the label "decisive" it's that.
Using PFD as of Grant crossing the Mississippi (based on the March 31 numbers):
Port Gibson has 4,654 Confederates.
Port Hudson has 16,287 Confederates, minus Buford's brigade (so -2,735 PFD for a true PFD of about 13,500)
Loring has 7,227 PFD, plus Buford's brigade (so near enough 10,000 PFD)
Across the upper Mississippi there's 2,824 PFD.
And at Vicksburg (exc. Port Gibson) there's 17,407 PFD.
There's 431 PFD at Jackson as well.
Port Hudson is being pressured by Banks (whose Department of the Gulf has 35,670) so the force at the point of contact is facing a department nearly three times as strong as it is.
The force across the upper Mississippi is forming a cordon against the 16th Army Corps. Their immediate tribulation is a potential move by the District of Corinth which is about 10,800 PFD, and of course the forces at Jackson and Memphis which sum to about 12,500 more PFD; perhaps most of that's not available for offensive operations but it's still enough that a cordon of less than 3,000 is vulnerable to be outnumbered.
The force Grant has that moves across the river itself in a single lift (spread over two days) is ca. 30,800 PFD when it makes contact around Port Gibson on the 1st; that's about a 6:1 superiority in numbers against Bowen's force.
And the total force of Grant's field army in three corps under Sherman, McClernand and McPherson (listed as "operating against Vicksburg" in the April 30 return) is about 47,000 PFD; adding the units individually reveals that at least one division besides the District of Eastern Arkansas was missed out of the "operating against Vicksburg" count and looking at artillery counts it seems to be the 14th division of the 13th Army Corps*. This division certainly fought at Port Gibson so I'm not sure why it's excluded; this brings the total PFD that should be counted as operating against Vicksburg to 52,000 PFD.
*the missing division has six field guns, which Carr had; additionally Carr had six heavy guns and the Grand Total Operating Against Vicksburg lists no heavy guns. Checking officer counts confirms it.
Grant's force operating against Richmond (before he brings up anyone from 16th Corps) at 52,000 PFD is slightly stronger than Pemberton's entire department (at 49,000 PFD), and Pemberton is also being pressured from the south by a separate department of considerable size (which as of March 31 is drawing off almost exactly a third of his troops) and from the north by a smaller force; under these circumstances the coordination required to obtain superior force at the point of contact is worthy of note but is also "how things should be done". With that superiority of force and with Grant holding the initiative, he can (and should, and indeed did) only move when the force concentration is right.
Unfortunately I can't find a map offhand that indicates where Loring was at the point Grant crossed...