I kinda tried to avoid a direct comparison in neck sizes between my Spector Rex and my Gibson Thunderbird because of the apples and oranges of comparing a 5 string size neck to a 4 string size neck. But since you seem genuinely interested I'll run up to the studio and grab some quick measurements of the two necks.
I'll measure the width of the nut, the thickness of the neck at the first fret, the width and thickness of the neck at the 12th fret the width of the neck at the 20th fret and the centerline to centerline of the outside strings at the bridge and string spacing at the bridge of each bass.
If someone with a Five string Thunderbird can chime in with actual dimensions of the same spots on their bass it would be very helpful.
Here ya go...
Spector Rex 5 Pro data first vs 4 string Gibson Anniversary Issue Thunderbird data
Nut width: 1.75" vs 1.5"
Thickness of neck at fret 1: .75" vs .75"
Thickness of neck at fret 12: .875" vs .875"
Width of neck at fret 12: 2.375" vs 2.125"
Width of neck at fret 20: 2.75" vs 2.25"
Centerline to centerline of Outside strings at the bridge: 2.75" vs 2.125"
String spacing at bridge: just shy of .75" vs just shy of .75"
Apparently both basses have the same string spacing and both basses have roughly the same neck thickness. The widths of the two basses fret boards is obviously different because of the number of strings. Both necks have a similar contour - D shaped vs a more rounded C shape.
Just to give you another point of reference - my Squire Affinity Jazz 5 has a 1.875" nut, is 2.5" across at the 12th fret, is 2.875" across at the 20th fret and is about the same thickness at the 1st fret and 12th fret as both the Spector and Gibson with a more rounded almost C shaped neck.
Hope this helps.