Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I create one? Using a reprex, complete with representative data will attract quicker and more answers. That will reduce community reverse engineering desire disorder. 
For the decimal question, I think the function is returning what it is supposed to, which is a precision of 1000 bits, not 1000 bytes representing digits.
suppressPackageStartupMessages(library(Rmpfr))
mpfr(pi,1000)
#> 1 'mpfr' number of precision 1000 bits
#> [1] 3.141592653589793115997963468544185161590576171875
Created on 2020-04-08 by the reprex package (v0.3.0)
For the second, I think you want to use
suppressPackageStartupMessages(library(Rmpfr))
mpfr("7f2d36a2a0007f2d36a2a0007f2d36a2a0007f2d36a2a000", base=16)
#> 1 'mpfr' number of precision 192 bits
#> [1] 3118361524223520784583964884878580812558070356334996529152
Created on 2020-04-08 by the reprex package (v0.3.0)
(Didn't have time to take it out to 10K on my puny Airbook)