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High-resolution simulations of star-forming massive galactic discs have shownthat clumps form with a characteristic baryonic mass in the range$10^7-10^8~M_{\odot}$, with a small tail exceeding $10^9~M_{\odot}$ produced byclump-clump mergers. This is in contrast with the observed kpc-size clumps withmasses up to $10^{10}~M_{\odot}$ in high-redshift star-forming galaxies. Inthis paper we show that the comparison between simulated and observedstar-forming clumps is hindered by limited observational spatial resolution andsensitivity. We post-process high-resolution hydrodynamical simulations ofclumpy discs using accurate radiative transfer to model the effect of ionizingradiation from young stars and to compute H$\alpha$ emission maps. By comparingthe intrinsic clump size and mass distributions with those inferred fromconvolving the H$\alpha$ maps with different gaussian apertures, we mimick thetypical resolution used in observations. We found that with 100 pc resolution,mock observations can recover the intrinsic clump radii and stellar masses, inagreement with those found by lensing observations. Instead, using a 1 kpcresolution smears out individual clumps, resulting in their apparent merging.This causes significant overestimations of the clump radii and, therefore,masses derived using methods that use their observed sizes. We show thatlimited sensitivity can also force observations to significantly overestimatethe clump masses. We conclude that a significant fraction of giant clumpsdetected in the observations may result from artificially inflated radii andmasses, and that $\approx 100$ pc spatial resolution is required to capturecorrectly the physical characteristics of star-forming clumps if they arecoherent structures produced by disc fragmentation.

Clumpy galaxies seen in H-alpha: inflated observed clump properties due to limited spatial resolution and sensitivity