Why does my diode log amplifier in LTspice not match theory?
You are using a model of a quasi-real diode, modelled with internal dynamic resistance, which changes with current. If you'll compare your output against an ideal diode, modelled with only Is=2.52n, and a behavioural source with that exact expression, you'll see this:

You can see how V(a) (with a 1N4148) is much lower, but also how its slope is different than V(b) (simple-modelled diode) or V(c) (behavioural expression). Notice how the ratio of V(a)/V(c) is a slope, which is proportional to the input (also a slope). That tells you why you're getting different results: that equation you see on Wikipedia is an ideal one, that does not account for all the possible effects that happen in a diode. Also, the reason you see slightly different traces between V(b) and V(c) is because of the rest of the default parameters for the diode, which are not all zero or infinite (you can see more in the help, at LTspice &gt; Circuit Elements &gt; D. Diode, 2nd table).