The amount of $^{56}$Ni produced in type Ia supernova (SN Ia) explosion is probably the most important physical parameter underlying the observed correlation of SN Ia luminosities with their light curves. Based on an empirical relation between the $^{56}$Ni mass and the light curve parameter $triangle m_{15}$, we obtained rough estimates of the $^{56}$Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived $^{56}$Ni masses for different SNe Ia could vary by a factor of ten (e.g., $M_{rm Ni}=0.1 - 1.3$ $M_{odot}$), which cannot be explained in terms of the standard Chandrasekhar-mass model (with a $^{56}$Ni mass production of 0.4 -- 0.8 $M_{odot}$). Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich (with $M_{rm Ni}$ $>$ 0.8 $M_{odot}$) SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more $^{56}$Ni to power the light curve. This is also consistent with the finding that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have $Delta m_{15} < 1.2$ while this ratio is only 18% in E/S0 galaxies. On the other hand, the nickel-poor SNe Ia (with $M_{rm Ni}$ $<$ 0.2 $M_{odot}$) may invoke the sub-Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.
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