I agree. It is better to finish a small project than start a big one and never finish it. I have seen a few people, and a few groups of people, who had big impressive plans... and never delivered. It is easy to imagine big: this game should be 3D, full of action, but also logical puzzles, and strategy, with an AI opponent, and also multiplayer; perfect graphics and a great storyline, but also allow complete freedom of movement... and then, five years later, you have five characters made in Blender, a map of the world where the story is supposed to happen, a list of fifty magic spells divided into multiple categories, and a tech demo where one of those characters is walking across an endless green plane.
If you can't make a small game, you can't make a big game. So make the small game first; it should take much less time than the big game, so if you are afraid that doing the small game would waste too much of your time, you are definitely not ready for the big game.
Even the small game requires a lot of work to make it look nice. Consider minesweeper: after implementing the minimum mechanism (click on a field to expose it, you either die or not, the number of adjacent mines is displayed), you are not even half done. You need the recursive exposure for fields with zero adjacent mines, editing the flags, showing the unflagged adjacent fields. Preventing the first click from being a mine. A hi-score table, where you can write your name, and it gets automatically saved, and loaded at start of the game. A menu to choose between simple, medium, and hard version of the game, maybe also custom dimensions. Should the game pause when minimized? All these details matter for user experience. Maybe also an installer?
On the other hand, if you can do the minesweeper right, you can easily create a new interesting game by tweaking some details. Maybe, play on a hexagonal map? Or play on an infinite map that scrolls to the right when you completely clear the leftmost column (except for the fields containing mines)? Add some bonuses (that you can collect by clearing a certain area) that allow you special moves, such as eliminating a mine, or reshuffling the existing mines? Would it be possible to create a version where you can have more then one mine per field? (Just a quick idea I had now, I don't even know whether that makes sense.) Maybe add some time pressure, like you need to make a click before the timer runs out, or maybe every twenty seconds a new mine is added to the plan (make sure it is not right under the user's cursor)? Or just make some non-functional feature, such as displaying a pretty picture in the background, and you get a new pretty picture when you complete the level.