What's the actual download size of Gimp?

Software in Ubuntu, when packaged, are usually split up into different packages, separating:

• architecture-independent data, (gimp-data)
• binaries, (gimp)
• shared libraries, (libgimp2.0)
• development headers, (libgimp2.0-dev)
• plugins, (gimp-data-extra)
• documentation, (gimp-help-en)
• and any external dependencies are usually separate packages as well.

The result is that the main gimp package depends on other gimp-related packages and many libraries as well. The download size of the main gimp package is, in fact, around 3 MB. The total download size, however, would be much more than 3 MB.

But you rarely see huge monolithic packages in the Ubuntu repository. 0ad-data, a particularly large package, is not even monolithic - it contains just the data files for the 0 A.D. game, and the binaries are in 0ad.

Packages from outside the repository, such as the Gitlab omnibus package (~300 MB!), can be huge because those developers decided to include every dependency in a single package instead of relying on repository packages.

As for the total download size, that depends on which libraries you already have installed on your system. apt will tell you how much will be downloaded:

$ sudo apt install gimp
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
  gimp-data libamd2.4.1 libbabl-0.1-0 libblas-common libblas3 libcamd2.4.1 libccolamd2.9.1 libcholmod3.0.6 libgegl-0.3-0 libgfortran3 libgimp2.0 liblapack3 libsdl1.2debian libumfpack5.7.1 python-cairo python-gobject-2 python-gtk2
Suggested packages:
  gimp-help-en | gimp-help gimp-data-extras python-gobject-2-dbg python-gtk2-doc
The following NEW packages will be installed:
  gimp gimp-data libamd2.4.1 libbabl-0.1-0 libblas-common libblas3 libcamd2.4.1 libccolamd2.9.1 libcholmod3.0.6 libgegl-0.3-0 libgfortran3 libgimp2.0 liblapack3 libsdl1.2debian libumfpack5.7.1 python-cairo python-gobject-2 python-gtk2
0 upgraded, 18 newly installed, 0 to remove and 0 not upgraded.
Need to get 17.2 MB of archives.
After this operation, 92.5 MB of additional disk space will be used.
Do you want to continue? [Y/n]

I recently learned that GIMP is a Photoshop replacement in Ubuntu world

That statement would cause a lot of flamewars. Both are image editing packages, both have strenghts and weaknesses.

I mean is that it or are there some hidden dependencies which are gonna eat my internet allocation away?

gimp is split into multiple packages, there are three that are essentially mandatory and a couple of optional ones. Those packages will also depend on other libraries that aren't part of gimp and may or may not be already installed.

The three mandatory ones are gimp, gimp-data and libgimp for a total download size (installed size will be bigger) of about 12 megabytes on top of this there will be libraries that aren't part of gimp but are needed by it the exact set will depend on what you already have installed.

then how can Linux apps be this small?

A couple of reasons, firstly linux distros try to share as much code as possible between different applications while vendor-distributed applications (for both Windows and Linux) tend to embed a lot of libraries.

Secondly a large part of the size of an image editing tool is not the core image editing functionality. It's all the extras and the value of a particular extra is only loosely related to it's size.

To start off, GIMP is hardly an analog of photoshop. Being the best open source product, does not mean you're comparable to the proprietary 600 lb gorilla.

I recently learned that GIMP is a Photoshop replacement in Ubuntu world and Photoshop that I knew was a huge application around 500MB

That's not even close to the size of GIMP. For example the GIMP packaged depends on the following,

Depends: libgimp2.0 (>= 2.10.6), libgimp2.0 (<= 2.10.6-z), gimp-data (>= 2.10.6), gimp-data (<= 2.10.6-z), libgdk-pixbuf2.0-0 (>= 2.27.1), xdg-utils, libaa1 (>= 1.4p5), libbabl-0.1-0 (>= 0.1.10), libbz2-1.0, libc6 (>= 2.27), libcairo2 (>= 1.12.2), libfontconfig1 (>= 2.12.6), libfreetype6 (>= 2.2.1), libgcc1 (>= 1:3.0), libgegl-0.4-0 (>= 0.4.8), libgexiv2-2 (>= 0.10.6), libglib2.0-0 (>= 2.55.2), libgs9 (>= 8.61.dfsg.1), libgtk2.0-0 (>= 2.24.10), libgudev-1.0-0 (>= 167), libharfbuzz0b (>= 0.6.0), libheif1 (>= 1.1.0), libilmbase23 (>= 2.2.0), libjpeg8 (>= 8c), liblcms2-2 (>= 2.8), liblzma5 (>= 5.1.1alpha+20120614), libmng2 (>= 1.0.10), libmypaint-1.3-0 (>= 1.3.0), libopenexr23, libopenjp2-7 (>= 2.0.0), libpango-1.0-0 (>= 1.29.4), libpangocairo-1.0-0 (>= 1.29.4), libpangoft2-1.0-0 (>= 1.29.4), libpng16-16 (>= 1.6.2-1), libpoppler-glib8 (>= 0.44.0), librsvg2-2 (>= 2.14.4), libstdc++6 (>= 5.2), libtiff5 (>= 4.0.3), libwebp6 (>= 0.5.1), libwebpdemux2 (>= 0.5.1), libwebpmux3 (>= 0.6.1-2), libwmf0.2-7 (>= 0.2.8.4), libx11-6, libxcursor1 (>> 1.1.2), libxext6, libxfixes3, libxmu6, libxpm4, zlib1g (>= 1:1.1.4)

Because Ubuntu is not a rolling distro, it's safe for the build of GIMP to assume that those versions are present on the system, when you go to install. However, Windows and like operating systems do not provide a repository to satisfy those dependencies; therefore, they can not make such bold assumptions. This means that the above libraries must be statically linked in, or they must be provided themselves.

Moreover, while all the libraries of above must be packaged by Photoshop on Windows, Adobe must further cover other issues,

Microsoft Windows 7 with Service Pack 1 (64-bit)*, Windows 10** (Version 1709 or later)

They have one download for 2 versions of Windows.