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tokens: their stellar mass by less than 10% across cosmic time. This result illustrates the uniqueness of these galaxies, suggesting a minimal contribution from merging events. Upcoming surveys such as WAVES ( Driver et al. 2016 ), covering wider regions of the sky, and deeper in flux than GAMA, will allow us to produce a high enough number of MUGs, to explore their formation channels in detail. It is remarkable that our sample of massive compact galaxies appear – at least in projection – in a relatively crowded environment (see Figs 1 and 2 ), complicating the photometric and spectroscopic analysis. Therefore, the inferred sizes and densities only provide upper bounds, as the light from the minor companions may contaminate the surface brightness profiles in a way that is impossible to mitigate at the spatial resolutions available to us. Follow-up observations, including imaging at high spatial resolution with HST or JWST, and deeper spectroscopy with IFU cameras at 8–10m-class telescopes, such as MUSE or MEGARA, will be required to better comprehend the intriguing properties of massive ultracompact galaxies. Appendix A: Robustness of the measured structural parameters We simulated 3840 galaxies, both in the KiDS r -band and the VIKING Z -band, with the tools described in Buitrago et al. (2013) . We explored the range of structural parameters shown by our sample, namely: where mag, R e , n , a r and p a stand out for magnitude (either in the r - or Z -band), effective radius, Sérsic index, axis ratio and position angle. The structural parameters of the mock galaxies were randomly distributed in a linear way, considering the full parameter space defined by the previously mentioned values. The mock galaxies were placed randomly on the KiDS and VIKING images, only imposing the condition that they should not overlap with any detection defined by the SExtractor segmentation map of each survey. Each mock galaxy was convolved with a representative PSF from those images, and analyzed with the same code we utilize to investigate the real galaxies. According to Figs. A.1 and A.2 , our ability to recuperate the structural parameters degrades at fainter fluxes and higher Sérsic indices. This result is expected, because dimmer and more concentrated objects are more difficult to analyze (a similar behaviour is found in the simulations of Buitrago et al. 2013 ). In the Z -band, the results are somewhat worse because of the coarser pixel scale (0.21 arcsec pix −1 versus 0.339 arcsec pix −1 ). However, for the data at hand, caution needs to be taken only for faint objects with a high Sérsic index. Fig. A.1. Relative errors – (output-input)/input – of the structural parameters (magnitude, effective radius and Sérsic index) of our simulated KiDS galaxies in the r -band. The right-hand column shows the average values in 1 mag bins (derived after a 5 σ clipping). The error bars represent the standard deviation within each bin. Fig. A.2. Relative errors –