Currently I carry the slim 9mm 3913. If I were to switch to the larger 45 caliber round, what would be the smallest manufactured auto available? One final thing. I do not like "cocked and locked" carry. Never have, it's a personal thing with me. Does like eliminate any ideas for 45 carry?
I've been on a quest for a tiny big-bore auto since the 80s. My first was a Detonics .45. It was totally reliable, but the special recoil springs required replacing regularly, and it shot very, very low from where the sights were pointed.
As far as modern guns which fit within your described parameters, I think you really have three choices:
The Glock 36 is a single stack, plastic frame, safe-action (striker-fired version of DAO) 6+1 45ACP. It's very accurate and very light. dano1200r has a set-up that is as minimalist as you will find. A belt clip and glock-block (trigger block to keep the trigger from inadvertently getting pulled - something imperative when not carrying a Glock in a holster) means no holster and NO BULK. This is the easiest-to-conceal set up and my preference.
The Kahr PM45 is something that has intrigued me, but I haven't gotten one yet. Since I've not shot or carried one, I'll defer to Catshooter, whom I know you can trust. If he says it works and works well, I'd take that to the bank.
The last option is a Para-Ordnance LDA Compact. I don't own one of these yet, but I have one on order for my wife in 9mm. I've also handled and shot the LDA extensively, and I can tell you that for a NON-single-action trigger it must be tried to be believed. It is the lightest, smoothest Double-Action trigger I've ever felt, and I never thought I'd say that a DA trigger would beat a tuned S&W revolver, but the LDA trigger does so. I've attached a pic of the LDA Compact 45 below, just so you can compare.
Most of the rest of the compact 45s are generally not something I would bet my life on. They're either cheaply-made or unreliable or both.
I will say, that if deep concealment is not an issue, such as in the winter or Spring/Fall jacket weather, I'd prefer something a tiny bit bigger than these micro 45s, just because they are significantly easier to shoot and generally a little more reliable. I like the CCO from Para, which is an Officer's size frame with a Commander sized slide.