You can unlock a Cheat called 'Twig Boy' then enable it in the secrets menu! This makes Mike very skinny!
Temple Of Fire - Pool of blood room with the two blood waterfalls and two different ramps going around the room. Go up the ramp and shoot the second panel to open the passageway. Go inside and then proceed up the big ramp. Jump on the railing to the left and jump towards a hallway in the wall. You should grab the ledge in front of it. Pull yourself up. The "Secret Activated" message will appear.
Alternatve way is to come back when you have the flambeau go to the top of the blood pool room and enter the passageway, use the flambeau on one of the skin doors then it will lead you to the cheat location.
PC and Dreamcast Locations:
Exclusive to the N64 version.