Laser wakefield acceleration of electrons is studied experimentally in the strongly mismatched regime. Up to 20 J, 60 fs laser pulses are focussed at the input of a gas cell with the f/46 focussing system. The resulting 44 µm focal spot at the cell entrance is about two times greater than the matched spot size. Experimental results are supported by 3D particle-in-cell (PIC) simulations of the laser–plasma dynamics as well as by numerical simulations of the gas distribution inside the gas cell. Special attention is paid to the accuracy of spectra reconstruction affected by low pointing stability of the accelerated electron beam. It is demonstrated experimentally that the electron energy in the mismatched regime can be higher than that in the matched one and can reach GeV level for the used laser parameters.