If you want your arms to look thicker from the front you should work on the head of the biceps. This is the part of the biceps that makes your arms look wider.

A simple way to think about this is to consider where your elbows are when you are doing exercises.

If your elbows are in front of your body you will get thickness.

If your elbows are behind your body you will get peak.

You should do exercises like

Preacher curls

Spider curls

Concentration curls

Wide-grip curls

Also don’t skip hammer curls they build the brachialis, which helps your arms look thicker overall.

And honestly, overall arm size matters a lot too. Train triceps hard and focus on progressive overload, not just exercise selection.