A lot will depend upon how close the maker is able to keep a standard formula ratio of water to cement (cement means Portland Cement: when mixed with water and aggregate it becomes concrete), and if the parts are able to pass through the heat cycle necessary to thoroughly cure the item. A small item having low mass won't generate enough heat to do it and may need to be helped by putting it into a black plastic bag and leaving it in the sun. The water in the mix can't be allowed to "dry" out, it evolves into the molecular structure if the heat necessary is present, hence the sun-heat-absorbing bag that keeps the moisture in. It should stay sealed in the bag for several days or a week and will still remain damp until all the water is used in the molecule building process. Cement itself is not very strong, the real strength lays in the aggregate, crumbly materials will fall apart and bigger particles, like gravel, is much stronger. A stone the size of a Tylenol tablet can displace hundreds of grains of sand. The stone doesn't need any cement to hold the stone together. The grains of sand of that same displacement needs to have cement in all the spaces between grains to make it a whole, dense mass, but that mass does not have nearly as good of bond as the molecules within the stone. Larger aggregate also contribute to a "wetter" mix because it takes less cement by the same volume as the volume of the stones. The mix only has to fill in the space between the stones and most of that will be the grains of sand. Typical roads use larger grains of sand, like #3 grade instead of 000 silica used in white motor. A good combination of sand sizes and larger stones (gravel) fills up more space with aggregate requiring less cement to bond it all together. You can see that without gravel, the concrete will not be as strong, and will use more cement and water to be plastic. The strength in concrete comes from the aggregate being held in place by the cement, not from the strength of the cement.