In the past I've read where Eheim says their slower flow rates than some similar sized filters makes for more efficient nitrification. Don't know how much difference it makes, really. I've had smaller, faster filters do the same work of larger, slower filters in any given tank and vice versa. My theory is a filter with twice the media volume and half the flow rate is roughly equal to half the media volume and twice the flow rate, and my experience roughly confirms this. The main difference in this scenario is the smaller filter will need cleaning more often.
No fish doesn't necessarily mean no bacteria bloom. The microbes in your water column that can cause cloudiness are a different type from those in your filter, they live differently and multiply much faster. There can still be minerals and other nutrients in your water that feed the water column microbes, which are capable of multiplying so quickly they become visible, hence the cloudy water. According to some articles, in a new tank these nutrients may include silica dust from new sand or other sources, besides whatever nutrients are in your tap water. In any case, water changes don't always fix it, not if the nutrients are in your tap water, and going overboard with water changes can perpetuate the problem or make it worse in some scenarios. It's also possible, no matter how well you rinse new sand that there's still dust coming from your sand and it just needs time to settle and filter.
Best solution ime is time, letting your tank balance out and you're new filter and media mature a bit is better than going nuts with water changes. Sometimes it just takes a while. The other alternative is micron filter polishing if you have the means to do it. In the meantime you can still add some fish if the bloom isn't extreme, as long as ammonia/nitrite is okay, some cloudiness doesn't normally bother them, unless it's an extreme bacteria bloom producing too much ammonia or exhausting oxygen. In fact, I wouldn't go weeks without fish-- unless you're ready to go through fishless cycling--- because your beneficial bacteria will begin to shut down you'd be going backwards.
