The software on your machine is extremely basic, and therefore unable to do more than minor adjustments to the loaded pattern; the reason behind this is that your embroidery file isn't just a pattern to be filled in, but a pattern of every point where the needle enters the fabric, at what point and in what order. Simply resizing without re-rendering will either move the stitches closer together (scaling down), which could become much too tight on some patterns, or move them farther apart (scaling up), leaving much more of the background fabric exposed than intended.
The commonly recommended scaling for patterns without re-rendering is no more than 10%; at this level, the issues with stitch spacing are less visible and easier to ignore. Above this level, specialized software is needed to handle adding or removing stitches--and even then, you still may not get the desired result and may find better results by purchasing a different size (if available). I personally have had better luck with scaling down on patterns using more basic software, as it tends to be a bit better at removing stitches than inserting them, but have largely now turned to buying the size closest to what I want and sticking to very small adjustments.
This blog has some more detailed information on the perils of resizing, including some helpful graphics to illustrate the potential issues, as well as information on using specific software to help get around these issues (and why it might be tricky to do so with pre-rendered designs).
With higher-end machines, the processing power is also increased, allowing these to do greater resizing steps, but even those still have their limits, similar to those of machines with lower memory and processing power. Some of the cautions from the manual for the Husqvarna Viking Designer Diamond machine include:
- "If the design is very detailed, some of it may be lost or distorted, and/or it can become very dense. Some designs should not be reduced more than 25% if they have too many details."
- "If you are enlarging a design too much, you may see stitch irregularities.... Depending on the memory left in the machine, you may find some designs too complex to resize in the machine. Please use ... Embroidery Software to resize very large and complex designs."
- "Resizing a very larger or complex design may take a few minutes. Once you touch OK, you cannot cancel."
(Source: Designer Diamond manual, page 7:8)
While this machine has the ability to select whether the pattern is re-rendered (and is able to do so onboard), the recommendations there are still to always re-render over 20%, with multiple cautions in the section about resizing using the onboarding software about the unpredictability and pitfalls of resizing a pattern. Even with this accessible, I still primarily opt to buy multiple sizes of a pattern to fit my needs better.