My understanding the difference is size mostly although the standard size will also lay slightly larger eggs.
Here’s how the I internet explains it…
The main difference is
size: standard Silkies are significantly larger and heavier, while bantam Silkies are a miniature version. While standard Silkies are the original European variety, in the US, all Silkies are often classified as bantams, though they are bred to different size standards. The smaller bantam size requires less food and space, while the standard size may offer slightly larger eggs and more meat.
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Feature
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Standard Silkie
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Bantam Silkie
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[td]Size[/td][td]Larger and heavier[/td][td]Smaller and more compact[/td] [td]Weight[/td][td]Approximately 4 lbs (1.8 kg) for males and 3 lbs (1.4 kg) for females[/td][td]Approximately 22 oz (600g) for males [/td]