Jason Bassford suggested lightweight for one end of the scale; for the other end, I'd suggest the obvious opposite: heavyweight.
That can carry connotations of size (installation, codebase, and/or UI/API), the complexity of what it does, the resources it uses (CPU, memory, storage, and/or servers), cost, level of robustness/redundancy, or even prestige/reputation.
Or you could simply describe a piece of software as big.
That's more specifically about aspects that can be measured, most usually the size of its installation.
And in response to some of the other answers and comments: in standard English ‘software’ is a mass noun (like ‘air’ or ‘bread’): you can talk about some software, or a piece of software, but you can't refer to ‘a software’ (nor to ‘softwares’). (Even though it might be more convenient to do so!)