int initialOffset = 10;
int additionalLength = BigArrays.BYTE_PAGE_SIZE;
byte[] expectedData = randomizedByteArrayWithSize(initialOffset + additionalLength);
// first create initial offset
out.writeBytes(expectedData, 0, initialOffset);
assertEquals(initialOffset, out.size());
// now write the rest - more than fits into the remaining first page
out.writeBytes(expectedData, initialOffset, additionalLength);
assertEquals(expectedData.length, out.size());