General - Photo editing advice

I'm trying to make a poster collage to send to a print shop and have it pasted on a wall... The print would be huge (around 100 inches tall to 150 inches wide), and so I wanted to put various poster images in one sole image to send it directly to the print shop..

Yet the problem I'm having is that when I try to create such a big image canvas, the image editor (Gimp) is telling me that such a size will consume 4.8GB of memory, that if I'm sure I want to proceed...

So I cancel and create a poster size image (around 100MB!!) and start adding large poster images and expanding the canvas size, and ideed the Gimp process starts consuming lots and lots of memory...

What I don't get is... If i'm adding 3MB to 10MB jpeg image files to the canvas, why is the size so huge? I know that jpeg is compressed, and that the image I'm working with is uncompressed and probably that's the thing... But the main question then is: how can I do this without breaking my pc? How can I create a collage of compressed JPEGs and obtain a simple (say) 30MB jpeg file??

To really control it I'd advise using photoshop - you need to be sure that 'all things are equal' in terms of resolution, sampling etc.

A very large, high-res image will be very large.

photoshop and gimp are not meant to do this. while editing a jpg in one of these programs, it gets decompressed, which consumes gigabytes of memory.

A possible solution would be the use of a program like Adobe InDesign

buy a new pc ! a better one.
I'm graphist and we use photoshop to make artworks which will be print on vinyl banners ( sometimes up to 10 meters) and usually our artworks are never up to 250 MB, we send the artwork file in pdf.

Should be more tools

thatll really be large

Graphic designer here also - Generally in my experience of large scale print - eg billboard it is of course scaled, but you do need it to be of high quality in the first place - InDesign is a layout tool, but it does not edit graphics in any way so you will not be able to ensure all images are same size/quality - it will allow you to construct your poster however.

Photoshop is your best bet - then talk to the printer and find out what type of printing they support - what resolution they advise for scaling and also ask them for a colour profile so you can wysiwyg the whole thing