Why do you think that?
There are lots of 5/8 oz loads in the load manuals
I was thinking the .410 slug is 1/5 oz. I found a interesting shot comparison on Wikipedia Comparison chart
Below is a chart with diameters per pellet and weight for idealized lead spheres.
Size Type Weight Diameter
#TriBall(12 Ga.) Buck 20.41 g (315 gr.) 15.24 mm (0.60")
#0000 Buck 5.51 g (85 gr.) 9.40 mm (0.380")
#000 Buck 4.54 g (70 gr.) 9.14 mm (0.360")
#00 Buck 3.49 g (53.8 gr.) 8.38 mm (0.330")
#0 Buck 3.18 g (49 gr.) 8.13 mm (0.320")
#1 Buck 2.62 g (40.5 gr.) 7.62 mm (0.300")
#2 Buck 1.91 g (29.4 gr.) 6.86 mm (0.270")
#3 Buck 1.52 g (23.4 gr.) 6.35 mm (0.250")
#4 Buck 1.34 g (20.7 gr.) 6.09 mm (0.240")
#FF Buck 1.18 g (18.2 gr.) 5.84 mm (0.230")
#F (TTT) Buck 1.05 g (16.2 gr.) 5.59 mm (0.220")
#TT Buck 0.98 g (15.1 gr.) 5.33 mm (0.210")
#T Buck 0.89 g (13.7 gr.) 5.08 mm (0.200")
#BBB 0.66 g (10.2 gr.) 4.82 mm (0.190")
#BB 0.57 g (8.8 gr.) 4.57 mm (0.180")
#B 0.48 g (7.4 gr.) 4.32 mm (0.170")
2 3.76 mm (0.148")
4 3.28 mm (0.129")
5 3.05 mm (0.120")
6 2.77 mm (0.109")
7.5 2.39 mm (0.094")
8 2.26 mm (0.089")
8.5 2.16 mm (0.085")
9 2.01 mm (0.079")
12 1.3 mm (0.05")