"小山似的一堆" can be translated into English as "a pile as big as a small hill" or, more naturally, "a mound-like heap" or simply "a hill-sized pile" (though the last one is a bit more figurative and less common in literal contexts).
The most straightforward and commonly used expression in a literal sense would be:
"a pile as big as a small hill" (emphasizes the size comparison)
"a mound-like heap" (more concise and descriptive of the shape and size)
For a more idiomatic or slightly poetic touch, you could also say:
"a heap resembling a small hill"
But in most everyday contexts, "a mound-like heap" would be perfectly adequate and natural.