There are a couple of issues that I can see here that may be limiting how your fish grow.
You didn't mention having a heater for the tank and I cannot see one in the picture that you have provided. Whilst guppies can survive in colder water, to make a fish thrive you want to provide all the chances you can.
A stable temperature is very important and warmer water around 25-28 °C (77-82 °F) tends to provide the best conditions
Source: Guppies and Mollies - thinkfish.co.uk
Even if you live in a climate that might allow the temperature to normally reside within this window, you'll probably want a heater for consistency.
2. Water Quality
Generally in my experience, a large volume of water is easier to keep at a higher quality because fluctuations caused by water changes have less impact on the aquarium's water quality parameters.
A filter is a must for a sustainable tank for a number of reasons. You'll want to check out the nitrogen cycle if you're not familiar with it already. To keep a tank sustainable for fish it must be 'cycled' so that you don't get a build up of toxic ammonia in your tank. If you haven't got yourself a water quality test kit you'll probably want to look into that too. The filter will also help to keep the water moving, agitating the surface, and allowing for more oxygen to dissolve in the water.
Ensure that you are doing regular water changes (normally between 15-25% a week) to keep the levels of nitrate down.
3. Aquarium Size
The tank you've got there looks pretty small. You should probably have at least 30 litres to give them a good enough space to live comfortably in. If you intend to bread them heavily, you'll need something much larger because they are prolific breeders.
As with most fish, the better you can set up their environment, the better they'll grow and the longer they'll live.
This probably looks like a lot of information. Well, I guess it is. The key points here are:
- Make sure the tank setup is suitable (size, temperature, water quality).
- Consistency is key (It's widely agreed that chancing the perfect parameters does more harm than good in the long run. Get a good setup going and then maintain it).
- Reach out! (The best thing you can do is to ask for help if you need it - as you have already. I would suggest checking out some basic fish keeping videos like these to get to grips with the area).