The most obvious is cost, The larger tanks are more expensive and hobbyist have even gone to the extreme of Home extensions with a fish room full of equipment. If you have got it then go for it! many have not. Small reef tanks are reasonably priced and newcomers can come in at all levels.
A small reef tank doesn't need all the fancy equipment and can be run easily provided good housekeeping is practised. But that said with fancy computers, reactors and sump. Everything can be run with very little fuss, in autopilot if you like, with weekly checks to make sure everything is running smoothly.
Here is where larger reef tanks win. parameters are more stable and swings in water changes are minimum and happen in a much longer time. A slight problem with a small reef can send readings high really quickly.
If you want to stock really big and fancy specimens you are going to need a big Aquarium. It's not fair to stock a Yellow tang or similar type of fish in a 100 litre tank. The same applies for many types of fish all in one tank.
If you have a little space in your living room and don't know what to do with it then a Aquarium is a great addition, But don't plan on a huge aquarium it will be just too much impact.
This is very important, what would be the point of creating a beautiful built in fish tank if you plan on moving, it will be costly, very time consuming and may even result in the loss of a sale of your property. Not everyone wants to buy and house with a aquarium the cant do nothing with.
Small tanks can be moved relatively easy but it is a timely job. I moved last year and it took me a day to move a simple 125 litre tank and livestock. It was very tiring but can be simply be done through planning and a little organisation. This could take weeks maybe a month are so with a really big Aquarium.
A Small reef tank cant even compete with this. How nice would it be to have a six foot fish tank in your living room full of fish corals and invertebrate's? We all want the WOW factor.
Will it fit through your front door, garden gate, can it be easily transported? you wont have these problems with a small aquarium.
There is nothing stopping you implementing all the technologies in a small reef tank as you progress. At least this way you will have it for the large Aquarium you are planning for the future.