'Heavy on' and 'heavy with' are usually not interchangeable, although I'm sure there are folks who use them interchangeably, as both expressions can be used to mean a large quantity of something.
'Heavy on' is usually used when emphasis is on the quantity.
'Heavy with' is usually used when emphasis is on the weight or burden.
The spaghetti sauce was heavy on the garlic. (lots of garlic)
Her sundae was heavy on the sprinkles. (lots of sprinkles)
The jeep was heavy on fuel. (it used a lot of fuel)
The jeep was heavy with fuel. (it weighed a lot)
The tree was heavy with snow. (its boughs were being weighed down)
She was heavy with child. (burden)
Her thoughts were heavy with the idea of having to perform better. (burden)
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