Resistance changes when keeping two ends of a metal bar constant but increasing the thickness of the middle?
The answer to this question is not quite as simple as suggested in the comments above.
All materials have a bulk resistivity measured in ohm-meters. From Wikipedia, we have:

Electrical resistivity (also called specific electrical resistance or
volume resistivity) and its inverse, electrical conductivity, is a
fundamental property of a material that quantifies how strongly it
resists or conducts electric current. A low resistivity indicates a
material that readily allows electric current. Resistivity is commonly
represented by the Greek letter ρ (rho). The SI unit of electrical
resistivity is the ohm-meter (Ω⋅m).12 For example, if a 1 m × 1
m × 1 m solid cube of material has sheet contacts on two opposite
faces, and the resistance between these contacts is 1 Ω, then the
resistivity of the material is 1 Ω⋅m.

For example, copper has a volume resistivity of 1.68 × 10−8 Ω⋅m at 20 °C.
Note the units. If you divide volume resistivity (Ω⋅m) by cross sectional area of a conductor (m²) you get Ω/m which is the resistivity of that conductor per meter.
Which is to say - if you increase the cross sectional area of the middle part of your conductor, you will decrease the resistance by some amount.

However this isn't the entire story.
For example, what is the resistance of the below conductor?

Another way of asking this is - when I apply a voltage across the ends of this conductor, where does the current flow?
The answer is that the current is not equally distributed across changes of cross sectional area (or, for that matter, at sharp bends in a conductor.) This is the &quot;stream line&quot; effect as discussed in the Copper Development Association publication Copper for Busbars, section 6.3.1 Jointing of Copper Busbars - Joint resistance - Streamline effect.


As with all decent engineering questions - the answer is ultimately &quot;it depends.&quot;