“Better than to exaggerate, I have lessened in all things.” - La Relacíon
On no evidence at all (I don't know Spanish) I suggest this is a clumsy translation of an original that says something like
Rather than exaggerating I have understated everything.
If we put the subject first we get:
I have lessened in all things, better than to exaggerate.
The parallelism and elision/sluicing (basically, omitting) is tricky, but the question is: what does the author mean by lessened?
This is the speakers way of saying, "Of everything I have told you, that is worst-case scenario; if you investigated my assertions, they would meet or exceed expectations. In my opinion, this is better than talking it all up (to exaggerate)"
To avoid any chance of exaggeration, he has purposely gone the other way and and "lessened" his statements.