I am suffering from arthritis in my thumbs. I believe that fitting phat tape to my bars might help. However it comes as either 2.5mm or 4.5mm.
I am tempted to try the 4.5 but am worried that it will make the bars too bulky or even too squishy. I am male with small to medium sized hands.
Can anyone give any advice on this matter?

My wife suffers from some Ulnar nerve trouble so we put the Bar Phat on her road bike. I liked the feel so much, I put it on mne too. I can't verify the thickness since it is under the tape and I don't remember. I'm pretty sure it is the thicker that was available at the time I bought it - about five years ago. It really helps take out some of the road harshnes.

regular foam tape is about 2.5 mm. So if you try to imagine what it would feel like with a double layer that's what you'd have with the 4.5 mm.

From doing different bar tapings I've found that I can alter the feel of the tape with tension. When wound on slightly tighter the foam feels more firm than when wound on with only a little tension. So as long as you start with it wound on with a lower but still adequite tension to make it lay in place well you can always peel it back and redo it with a little extra tension to firm it up if you find it too squishy.

Think cushioned gloves instead. Get it right, and the same gloves will work on every bicycle in your fleet, and last you a long time.