Serious Sam TFE has a dynamic crosshair. Meaning it adjusts in size depending on what it's pointed at. This was somewhat common at the time with a few titles but very quickly died out. Nowadays it just feels weird and awkward. Thankfully there's a way to disable it.

Step 1. Pull up the console by pressing `













Step 2. Type in the following commands. Type in each one individually and press enter when you're done.
After that you're done.

That's about it. That's how to remove the dynamic sizing from the crosshair. If you want to adjust how big the crosshair is, just adjust the 0.5 in the command /hud_fCrosshairScale to whatever you want.