Picosecond pulses at 1.56 micro mm wavelength are directly amplified with a diffraction limited beam quality in a core-pumped Er fiber with an 875 micro m(2) effective area. The interplay between nonlinear spectral broadening and anomalous fiber dispersion compresses the input pulse duration during amplification so that 42 nJ energy pulses with approximately 65 kW peak power are achieved without pulse break-up.